I remember I once read in Kant's "Kritik der Reinen Vernunft" about the tendency to come up with or invent new terms/words, about which he was quite "critical" (pun intended). I need the exact quote, but I am unable to find it. Can anyone help?
You are probably referring to the first two paragraphs of the first book of the Transcendental Dialectic (A312-13/B369). Here they are, in Guyer's translation:
"In the great wealth of our languages, the thinking mind nevertheless often finds itself at a loss for an expression that exactly suits its concept, and lacking this it is able to make itself rightly intelligible neither to others nor even to itself. Coining new words is a presumption to legislate in language that rarely succeeds, and before we have recourse to this dubious means it is advisable to look around in a dead and learned language to see if an expression occurs in it that is suitable to this concept; and even if the ancient use of this expression has become somewhat unsteady owing to the inattentiveness of its authors, it is better to fix on the meaning that is proper to it (even if it is doubtful whether it always had exactly this sense) than to ruin our enterprise by making ourselves unintelligible.
For this reason, if there perhaps occurs only one single word for a certain concept that, in one meaning already introduced, exactly suits this concept, and if it is of great importance to distinguish it from other related concepts, then it is advisable not to be prodigal with that word or use it merely as a synonym or an alternative in place of other words, but rather to preserve it carefully in its proper meaning; for it may otherwise easily happen that when the expression does not particularly occupy our attention but is lost in a heap of others having very divergent meaning, the thought which it alone can preserve may get lost as well."
He proceeds to illustrate this recipe by reinterpreting Plato's "ideas" in his own vein, as archetypes for objects of possible experience. But, as far as Plato's speculative use of the ideas goes, Kant says: "I cannot follow him in this, just as little as I can in the mystical deduction of these ideas or in the exaggerated way in which he hypostatized them, as it were; although the lofty language that served him in this field is surely quite susceptible of a milder interpretation, and one that accords better with the nature of things". The moral, apparently, is that people of past and present reach out for the "same" concepts imprinted in our reason, and it is better to reappropriate the words they were using, to clean up and sharpen their meanings, than to invent new ones.
It should be said that Kant's recipe would be controversial today, mostly because few now believe in the universally imprinted concepts that accord with the nature of things, and it seems to presume to legislate what other people meant, or should have meant. Also, Kant did quite a bit of the coining of new terms himself, his own advice notwithstanding. Noumenon, abderitism, pacific union, and the transcendental unity of apperception are some examples. He also inverted the traditional use of "subject", which was a rather radical "interpretation" of what Aristotle meant by to hypokeimenon, and which was closer to what we now call object. Hence the ambiguous use we have today.
See KRV, Transcendental Dialectic. Bk.I : Of Ideas in General :
To coin new words is a pretension to legislation in language which is seldom successful [...].