1. A sufficient condition of something else.

2. A condition that may not be sufficient for another, but can be seen a priori to provide good evidence for it. Thus the fact that someone is behaving appropriately may not guarantee logically that they are he/she is in pain, but it may be a priori true that it is excellent evidence for it. The latter usage is attributed (controversially) to Wittgenstein. It fits with a generally holistic view of language, with assertions tied to each other by semantic forces of different strengths, rather than by straightforward logical relations.