1. reason - the activity of reasoning and the faculty of reasoning (opposed to intellectus, the faculty of superior and intuitive knowledge).
2. ratio formalis, formal ground (constituted by the essential attributes of a thing as they are in our mind or in its definition); ratio rei, essence of a thing.
3. definition, concept, sense, signification.
4. ground of being.
5. cause.
6. reasoning, argumentation.
7. judgement.
(Lexicon Latinitatis Medii Aevi, Brepols 1975)

ratio, lat. die Vernunft, der Grund, neuerdings nur noch für das Denken des Verstandes, in dem alle emotionalen und intuitiven Element unterdrückt sind, gebraucht (diskursiv, Reflexion);
ratio fiendi, Grund des Seins und Werdens (Realgrund),
ratio agendi, Grund des Handelns (Motiv),
ratio cognoscendi, logischer Grund (Erkenntnisgrund).
Dazu rational, vernünftig, im Gegensatz zu empirisch, soviel wie aus der Vernunft stammend, durch Verstandesdenken gewonnen.
(J. Hoffmeister: Wörterbuch der Philosophischen Begriffe, Meiner Verlag, 1955)

rational. 1. Opposed to irrational. 2. Opposed to non-rational or arational. Those who have spoken of man as the rational animal have of course been employing the word in the second sense, meaning capable of either rationality or irrationality, not trying to make it true by definition that everything in fact achieves the one rather than the other.
rationalism. 1. In a narrow sense, the doctrines of a group of philosophers of the 17th and 18th centuries, whose most important representatives are Descartes, Spinoza, and Leibniz.
The characteristics of this kind of rationalism are:
a) the belief that it is possible to obtain by reason alone a knowledge of the nature of what exists;
b) the view that knowledge forms a single system, which
c) is deductive in character; and
d) the belief that everything is explicable, that is, that everything can in principle be brought under the single system.
2. In a wider sense, the term used to refer to the views of philosophers who accept only (b) and (d), that is, the thesis that everything is explicable in terms of one system.
3. In the most popular sense, a) the rejection of religious belief as being without rational foundation, or b) more generally, a commitment to reason as opposed to faith, prejudice, habit, or any other source of conviction considered to be irrational.
(Antony Flew: A Dictionary of Philosophy, St. Martin's Press 1979)

see also Rational Thinking, Rational Mind, Rational Knowledge.

A general faculty, common to all or nearly all men, and sometimes regarded, either seriously or by poetic licence, as a sort of impersonal external power ('the dictates of reason'). This faculty has seemed to be of two sorts, a faculty of intuition by which one 'sees' truths or abstract things ('essences' or universals, etc.) , and a faculty of reasoning, i.e. passing from premises to a conclusion (discursive reason).
Kant contrasts reason, which is concerned with mediate inferences, and understanding and power of judgment, which are concerned with acquiring concepts and passing judgments, respectively.
Practical reason has been distinguished from theoretical or speculative reason since Aristotle, and raises problems: Is reason in the practical sphere 'the slave of the passions' (Hume), i.e. is it limited to showing us means to ends which the passions dictate?
(A.R. Lacey: A Dictionary of Philosophy, Routledge, 1986)

The general human 'faculty' or capacity for truth-seeking and problem-solving, differentiated from instinct, imagination, or faith in that its results are intellectually trustworthy - even to the extent, according to rationalism, that reason is both necessary and sufficient for arriving at knowledge.
(Ted Honderich: The Oxford Companion to Philosophy, Oxford University Press 1995)

Reason is much more subtle than intellect. Reason is naturally joined up with intuition, that is with the holistic way of seeing and knowing. A reasonable man also acts reasonably, otherwise he could not be called reasonable. Reason is insight into intuitional truths and thus naturally affects the behavior, otherwise the insight would not have been total in scope.

- A thought, idea, or opinion formed or a remark made as a result of meditation.
- consideration of some subject matter, idea, or purpose.
(Merriam Webster's Collegiate Dictionary, Tenth Edition)

In Latin reflectere and reflexio mean 'to bend back' and 'bending back'. Animum reflectere, literally 'to bend back the mind', originally meant 'to turn one's own or another's mind away, dissuade from (a course of action)', but later came to mean 'to turn one's thought to, reflect on, something'. In the sixteenth and seventeenth centuries these gave rise to the German reflektieren ('to reflect') and Reflexion. These have three main senses, acquired by their Latin originals in medieval times:
(1) To bend back or reflect, e.g. sound, heat, and especially, light.
(2) To reflect on, consider a matter.
(3) To turn back one's thoughts or attention from objects to oneself, to reflect upon oneself. In Locke and Leibniz, 'reflection' is perception of oneself or attention to what is 'in us'.
(Michael Inwood: A Hegel Dictionary, Blackwell, 1992)

Kant refines Leibniz's account [of perception and reflection] by dividing self-consciousness or apperception into apprehension and reflection, with the former denoting consciousness of the receptivity of a perception and the latter consciousness of the spontaneity of conception. Apprehension is described as 'empirical' apperception characterized by consciousness of the 'inner sense', while reflection is 'pure' apperception and is characterized by consciousness of the understanding.
(Howard Caygill: A Kant Dictionary, Blackwell, 1995)

Reflection, however, always presupposes a standpoint outside of the reflected matter. You have to stand above the rational mind to think over the rational mind. In order to analyze the mind you must necessarily transcend the boundary of the mind, otherwise it is not possible to say something meaningful about the mind. (see Essay The Infinity of Thought)

A generic term that broadly refers to theories of perception wherein the sensing mind is believed not to have direct acquaintance with its objects, but to apprehend them through the medium of ideas that are supposed to represent those objects.
(Antony Flew: A Dictionary of Philosophy, St. Martin's Press 1979)

In Critique of Pure Reason Kant defines representations as 'inner determinations of our mind in this or that relation of time' (A 197/B 242). ... Representations with consciousness are entitled perceptions, and these are divided into sensations, or those 'which relate solely to the subject as the modification of its state' and 'objective perceptions' or cognitions (A 320/B 376)..... while the intuition provides a field within which the manifold of intuition may appear as a representation, it is the concept which synthesizes these representations into experience and knowledge.
(Howard Caygill: A Kant Dictionary, Blackwell, 1995)

Vorstellung (lat. repraesentatio), das im Bewusstsein auf Grund von vorhergehenden Sinneswahrnehmungen und Empfindungen zustande kommende "Bild" eines Gegenstandes oder Vorgangs der Aussenwelt, das auch ohne Gegenwart des Vorgestellten (Erinnerungsvorstellung) als mehr oder weniger vollständig reproduziertes Wahrnehmungsbild oder durch eine subjektive Verbindung von Wahrnehmungsbestandteilen früherer Wahrnehmungen erzeugt werden kann. Vorstellung ist insofern verschieden von Gedanke, Begriff und Idee, die auf nichtsinnlicher Erfahrung beruhen. Im weiteren Sinne ist eine Vorstellung jeder als ein relative Ganzes erscheinende Bewusstseinsinhalt, der auf einen intendierten Gegenstand bezogen wird, ohne Rücksicht darauf, ob diesem ein wirkliches Objekt entspricht oder nicht.
(J. Hoffmeister: Wörterbuch der Philosophischen Begriffe, Meiner Verlag, 1955)