A phrase is a substantial musical thought usually ending with a harmonic, melodic, and/or rhythmic cadence. The presence of a cadence distinguishes a phrase from a motive (or motif — a short melodic or rhythmic figure that recurs in a composition; a theme). Phrases are frequently four measures long, but may be longer or shorter; however, it must present a complete (but perhaps dependent) musical thought.
Two phrases may form a period — a beginning, continuation, and ending to a musical thought.
Source: Music in Theory and Practice (book)