Full Idea
The word 'transcendental' signifies not a relation of our cognition to things, but only to the faculty of cognition.
Gist of Idea
'Transcendental' concerns how we know, rather than what we know
Source
Immanuel Kant (Prolegomena to Any Future Metaphysic [1781], 4:293), quoted by A.W. Moore - The Evolution of Modern Metaphysics 5.4
Book Reference
Moore,A.W.: 'The Evolution of Modern Metaphysics' [CUP 2013], p.121
A Reaction
This is the annoying abduction of a word which is very useful in metaphysical contexts.