In music, stretto essentially means the overlapping of the same melodic idea/subject in two or more voices. Stretto can thus be considered a canonic technique, although it is usually applied only to a particular passage or passages within a piece that is not itself a canon.
Stretto occurs typically in fugues.
Paul Walker gives the following definition in the context of fugue: “the procedure of beginning a second statement of the subject before the preceding statement has finished, so that the two overlap” (P. Walker: Stretto (i), Grove Music Online ed. L. Macy (Accessed 12 December 2012), http:// www.grovemusic.com).
Note that the word stretto means “tight” or “squeezed” in Italian; the verb is stringere, the past participle stretto.