A sanskrit word meaning literally: "measure"
This refers more usually to a standard of reasoning, or criterion of truth.
Sources or bases of knowledge may be distinguished (note, not divided) into numerous forms (note, not parts), including:
You can think of western logical truth as being a pared down version of this, and the notion is intriguing in that it contains several concepts not even present in currently extant western logic. EG. the notion of negative perception, which is inadequately explained as being that our senses of the world give us a penumbra of knowledge as to what is not present in the world as well as what is. Like we know what's not supposed to be in the universe as well as what is already there.
As in all things, the Indians vary as much with each other as with the outside world, and differing schools of thought accept or reject these modes of thinking to differing degrees. A yoga teacher told me that whereas other schools such as the Mimamsa accepted them all, his school accepted 'perception, inference, testimony only', Vaisheshika (and I think Buddhism) accepted only 'inference and perception', and there was even one school called Carvaka that only took 'perception'.