Full Idea
The word 'transcendental' does not mean something that goes beyond all experience, but something which, though it precedes (a priori) all experience, is destined only to make knowledge by experience possible.
Gist of Idea
'Transcendental' is not beyond experience, but a prerequisite of experience
Source
Immanuel Kant (Prolegomena to Any Future Metaphysic [1781], 373 n)
A Reaction
One of two explanations by Kant of 'transcendental', picked out by Sebastian Gardner. I think the word 'prerequisite' covers the idea nicely, using a normal English word. Or am I missing something?
Book Reference
Kant,Immanuel: 'Prolegomena to Any Future Metaphysic', ed/tr. Lucas,Peter G. [Manchester UP 1971], p.144
Related Idea
Idea 21442 'Transcendental' cognition concerns what can be known a priori of its mode [Kant]