overture (n.)

mid-13c., "an opening, an aperture;" early 15c. as "an introductory proposal, something offered to open the way to some conclusion," from Old French overture "opening; proposal" (Modern French ouverture), from Latin apertura "opening," from aperire "to open, uncover" (see overt).

The orchestral sense of "a movement serving as a prelude or introduction to an extended work" in English is recorded from 1660s.