Transcendental idealism is a position in epistemology from Immanuel Kant. Kant argued that experience is fundamentally separate from reality, and that our knowledge is subjective — that knowledge is of the world as we experience it, rather than of the objective world.
Transcendental idealism differs from the metaphysical view of pure idealism (or German idealism, as it is sometimes called), in that it does not deny the existence of the outside world, suggesting that only minds exist. Instead, Kant's view is that the actual, objective world is unintelligible to the point where it is meaningless to talk about it. Rather than experience that world directly, the mind gathers disconnected sense data and renders it into the world as it appears to us.
Kant refers to objects in reality, outside of the mind, as things in themselves, and says that we have virtually no knowledge of these objects. What we do know, however, is the world that our minds have created by ordering sense data into time and space in a way that we can understand and operate within.
The opposing view to Kant's version of idealism is known as realism — that we are able to know the world as it is, that we can acquire knowledge about a thing in itself directly.