The tripartite definition of knowledge is perhaps the oldest accepted definition of knowledge. The tripartite definition holds that knowledge is justified true belief. (Specifically, knowledge in this sense is propositional knowledge — knowing that something, rather than knowing how or knowing someone.
The purpose of the tripartite definition is to provide an adequate account of our conception of what knowledge is. Specifically, it claims that knowledge is a sort of belief, that this belief is of something true, and that one must have a good reason for holding such a belief.
This definition dates back to Plato, and remained largely unchallenged until Edmund Gettier published a short paper in 1963 which provided supposed counterexamples to the justified true belief account of knowledge.
Gettier's counterexamples brought about several responses, which either sought to defend the tripartite definition, to amend it with new conditions, or to discard it in favour of a new theory of knowledge.
The standard defense of the tripartite definition is to say that justification is infallible — that no part of one's justification for believing something must depend on some false or unknown belief. This prevents Gettier-like counterexamples, but presents even bigger challenges with certain things that are generally considered to be knowledge are ruled out.