Full Idea
We have proved that everything intuited in space or time, hence all possible objects of experience, are nothing but appearances, mere representations, which ...have outside our thoughts no existence grounded in itself. I call this Transcendental Idealism.
Gist of Idea
Everything we intuit is merely a representation, with no external existence (Transcendental Idealism)
Source
Immanuel Kant (Critique of Pure Reason [1781], B519/A491)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.511
A Reaction
It is only 'transcendental' idealism because it is what can be learned from deconstructing our own cognition, while remaining neutral (I assume) about whether the things-in-themselves are mere ideas. He is notoriousy ambivalent.