Full Idea
A 'warrant' refers to those processes which produce belief 'in the right way': X knows that p iff p, and X believes that p, and X's belief that p was produced by a process which is a warrant for it.
Gist of Idea
A 'warrant' is a process which ensures that a true belief is knowledge
Source
Philip Kitcher (The Nature of Mathematical Knowledge [1984], 01.2)
Book Reference
Kitcher,Philip: 'The Nature of Mathematical Knowledge' [OUP 1984], p.17
A Reaction
That is, a 'warrant' is a justification which makes a belief acceptable as knowledge. Traditionally, warrants give you certainty (and are, consequently, rather hard to find). I would say, in the modern way, that warrants are agreed by social convention.