Individuation is "The determining of what constitutes an individual."
Ted Honderich: The Oxford Companion to Philosophy

The principle of individuation (principium individuationis) is "a principle that uniquely identifies one individual."
Antony Flew: A Dictionary of Philosophy

C.G.Jung defines individuation as follows: " Individuation means becoming an 'in-dividual', and, in so far as 'individuality' embraces our innermost, last, and incomparable uniqueness, it also implies becoming one's own self. We could therefore translate individuation as 'coming to selfhood' or 'self-realization' .
Relations between the Ego and the Unconscious, in The Portable Jung

For Schopenhauer, the principium individuationis is time and space: " ....I shall call time and space the principium individuationis,.... For it is only by means of time and space that something which is one and the same according to its nature and the concept appears as different, as a plurality of coexistent and successive things .
The World as Will and Representation, Dover, (section)23

In my theory of the Universal Mind (Hyponoesis), individuation is a similar process as described above, except that the individuating agency is the Universal Mind that manifests itself as Individual Minds (Exonoesis). This process of concretization or concrescence (to use Whiteheadian terms) must be resolved again in order to come to true and holistic understanding (transrational apprehension). As long as our mind remains within the inherent limitations of its individuality, we will not be able to grasp concepts and ideas that transcend individuality. Only be de-individuating our own mind, by transgressing the restraining individuality of our mind, is it possible for us to gain the original unity of the Universal Mind and from this point of view to understand transrationally the whole and its interactions. Deindividuation is not a process of denying or repudiating one's individuality, but only transcending it in thought. We are and remain individuals as long as we exist as these individual mind-body entities. Being an individual mind-body entity does not mean we are condemned to a life-long imprisonment. Every Individual Mind contains latently the power to regain its original unity with the Universal Mind. Since individual Minds are only manifestations of the Universal Mind, we are the Universal Mind, at least potentially. The actualization of our Universal Beingness consists in the process of deindividuation.

Intelligence, mind. Of thought, including practical thought. Plato: " the converse of the soul with itself, without speech, is what we called thought [dianoia]." Also some specific thought or intention of an individual.
J.O. Urmson: The Greek Philosophical Vocabulary, Duckworth 1990

In the analogy of the Divided Line, Plato explains the difference between reason [dianoia] and intelligence [noesis] as the highest level of the mind:

But you want to distinguish that part of the real and intelligible which is studied by the science (episteme) of dialectic as having greater clarity than that studied by what are called "sciences" (technai). These sciences treat their assumptions as first principles and, though compelled to use reason [dianoia] and not sense-perception in surveying their subject matter, because they proceed in their investigations from assumptions and not to a first principle, they do not, you think, exercise intelligence [noesis] on it, even though with the aid of a first principle it is intelligible. And I think that you call the habit of mind of geometers and the like reason but not intelligence [noesis], meaning by reason someting midway between opinion [doxa] and intelligence.

Two fundamental powers of our mind are intellect and reason, as elaborated by Plato in his metaphysics of mind:

Originally, the Greek distinguished between knowledge as deduced from rational or scientific thinking (dianoia, intellect) and knowledge derived from pure reasoning (noesis, reason). The intellectual knowledge belongs to the lower part of the KOSMOS NOETOS, that is, to scientific and mathematical objects, while the intuitional knowledge of the NOESIS is based on the unchangeable and eternal IDEAS.

The double aspect theory is the theory that mental and physical are two properties of some underlying reality which is intrinsically neither mental nor physical. It follows that it is incompatible with dualism, idealism and materialism because it includes the denial of mental substance and the denial of physical substance. It has it in common with idealism and materialism, however, to be a monism rather than a dualism because it includes the thesis that only one substance, or only one kind of entity, fundamentally exists.
Stephen Priest: Theories of the Mind, Penguin 1991, p. 150

...there exists in the universe only one substance (unam substantiam), and that is absolutely infinite... (Part I, Prop. XIV, Coroll. I)
It follows, that an extended thing (rem extensam) and a thinking thing (rem cogitantem) are either attributes of God or modifications of attributes of God... (Part I, Prop. XIV, Coroll. II)
Thought (cogitatio) is an attribute of God, or, God is a thinking thing. (Part II, Prop. I)
Extension (extensio) is an attribute of God, or God is an extended thing. (Part II, Prop. II)
...whatever can be perceived by infinite intellect (infinito intellectu) as constituting the essence of substance, appertains to one substance alone; and consequently thinking substance (substantia cogitans) and extended substance (substantia extensa) are one and the same substance, which is now comprehended through this and now through that attribute. Thus also a mode of extension and the idea of that mode are one and the same thing, but expressed in two ways... (Part II, Prop. VII, Scholium)
Spinoza: Ethics

Mind (Exonoesis) and Matter (Exohyle) are aspects or attributes of an underlying and fundamental entity (Hyponoesis). The problem of identity of each aspect is resolved in the fact that both are dependent on Hyponoesis for their existence and essence.
In Essay Unity and Plurality of Hyponoesis I discuss the philosophical problem of the one and the many (unity and plurality), introducing the theory of bi-aspectuality:

According to the dual aspect theory, both matter and mind are two different modalities or aspects of one underlying reality. I call this fundamental reality: Universal Mind (hyponoesis). There is no independent substance of matter or mind besides the Universal Mind. Therefore, aspectuality just means a different manifestation of the same reality.

The theory that mind and matter are two distinct things. Its most famous defender is Descartes, who argues that as a subject of conscious thought and experience, he cannot consist simply of spatially extended matter. His essential nature must be non-material, even if in fact he is intimately connected with his body. The main argument for dualism is that facts about the objective external world of particles and fields of force, as revealed by modern physical science, are not facts about how things appear from any particular point of view, whereas facts about subjective experience are precisely about how things are from the point of view of individual conscious subjects.
Ted Honderich: The Oxford Companion to Philosophy, Oxford University Press 1995

A theory concerning the fundamental types into which individual substances are to be divided. It asserts that substances are either material or mental, neither type being reducible to the other.
Antony Flew: A Dictionary Of Philosophy, St. Martin's Press, 1979

Dualism is the theory that two and only two kinds of a substance exist: minds and physical objects. A mind is a purely mental, non-material or spiritual substance, and a physical object is a purely material, non-mental, spatially extended substance. It logically follows that no mind is a physical object and no physical object is a mind. A person, on the dualist account, comprises both a mind and a body, but most dualists maintain that a person is essentially his or her mind but only contingently his or her body, or, to put it another way, a person is his or her mind but a person has or owns his or her body.
Stephen Priest: Theories of the Mind, Penguin 1991, Chapter 1

In Essay Mind and Brain Relationship I distinguish between a strong and a weak form of dualism:

On the contrary, dualism, as postulated by Descartes, means an assumption of two totally independent and different substances, although they are capable of interacting somehow. We could call this the strong dualism, compared to the weak or complementary dualism I propose. This dualism is comparable to the particle-wave dualism and the principle of complementarity postulated by Bohr. Both, particle and wave are an aspect of the same subatomic event. They complement each other and belong necessarily together. Mind and body, therefore, are not antagonistic or completely different. Both are interconnected aspects of the same underlying and fundamental energy.

In Essay The Identity of Subject and Object I reject Cartesian dualism on grounds of intrinsic contradiction:

The Cartesian dualism posits the subject and the object as separate, independent and real substances, both of which have their ground and origin in the highest substance of God. Cartesian dualism, however, contradicts itself: The very fact, that Descartes posits the "I", that is the subject, as the only certainty, he defied materialism, and thus the concept of a "res extensa". The physical thing is only probable in its existence, whereas the mental thing is absolutely and necessarily certain. The subject is superior to the object. The object is only derived, the subject is the original. This makes the object not only inferior in its substantive quality and in its essence, but relegates it to a level of dependence on the subject. The subject recognizes that the object is a "res extensa" and this means, that the object cannot have essence or existence without the acknowledgment through the subject. The subject posits the world in the first place and the subject is posited by God. Apart from the problem of interaction between these two different substances, Cartesian dualism is not eligible for explaining and understanding the subject-object relation.

Another term I use for the same idea is "subject-object schematism" (see Essay Nature and Development of Transrational Thinking:

Whenever philosophers and thinkers of all ages discuss thinking, they always assume an object-subject relationship as being the intrinsic and necessary feature of thought or mind. There is on the one side the subject who thinks and on the other side the object of thinking, that which is thought of or about. I think that this subject-object schematism is arbitrarily or unconsciously projected upon the thinking process.

This subject-object schematism is a useful instrument of our mind when handling everyday situations of our life. When applying philosophical or Transrational Thinking, however, this intrinsic dualism has to be overcome in order to grasp the fundamental unity of reality.

In my theory, the duality of subject and object, of mind and matter, of self and world is created by the mind itself. I postulate a form of mentalism in Essay The Identity of Subject and Object:

Since the object is only created by the activity of the subject, and the subject is not a physical entity, but a mental one, we have to conclude then, that the subject-object dualism is purely mentalistic.

see also entries for Subject and Object.