Wikipedia gives us the best definition:
A menhir (French, from Middle Breton: maen, "stone" and hir, "long"), standing stone, orthostat, lith or masseba/matseva is a large upright standing stone. Menhirs may be found singly as monoliths, or as part of a group of similar stones. Their size can vary considerably, but their shape is generally uneven and squared, often tapering towards the top.
More about Menhir, here's the Wikipedia link.