1.NounA partially ordered set that either has a join (a least upper bound) for any nonempty finite subset (a join-semilattice or upper semilattice) or has a meet (or greatest lower bound) for any nonempty finite subset (a meet-semilattice or lower semilattice). Equivalently, an underlying set which has a binary operation which is associative, commutative, and idempotent.