A Half Cadence
is one of a few deliberately unresolved cadences, characterized by ending a musical phrase (rarely a piece) on a V chord. Here, the leadup is unimportant; the chord could be a I, a ii, a IV, a vi, or anything else, so long as it then progresses to a V.
A special form of the Half Cadence
is the Phrygian half cadence, which is created through an iv6 - V progression, so named for its similar sound to the Phrygian mode.