1. The repetition in a second voice or part of a theme, motif, or phrase presented by a first voice or part. The first presentation of the theme is called the antecedent, the answer is called the consequent, and is usually a different pitch from the antecedent. The antecedent usually continues while the consequent is coming in, and the consequent need not imitate the antecedent exactly, but may vary the tune slightly.
2. The imitation by music of natural sounds such as bird songs, rain, thunder, etc.

SUGGESTED LISTENING EXAMPLES:

Imitation:
Bach: The Art of the Fugue, BWV 1080, Contrapunctus I

W. W. Norton - 4-CD Musical Example Bank -- Disc 1, Track 25

imitation, continuous:
Josquin Desprez: Ave Maria . . . virgo serena

W. W. Norton - 4-CD Musical Example Bank -- Disc 2, Track 43