The world as we have access to it in sense experience. According to Kant our “theoretical,” in contrast to “practical,” knowledge is limited to the realm of sense experience. Kant apparently thought that the true nature of the world external to the sensing agent was not accessible to beings limited in their sentient existence to the sensible world, or world of “phenomena.”
Kant held that we cannot know about “things-in-themselves,” but only about the ways in which they appear under the a priori forms of reason, for example the temporal and spatial dimensions that we impose on our sense experience in order to make it intelligible.
From A Dictionary of Philosophy (Rev. 2nd ed.) ed. Antony Flew (St. Martin’s, 1979), 325.