imitative polyphony

As an example: One singer begins singing a line of lyrics. Another begins four beats later, singing the same words, but not in unison with the first singer. The pitches sung may be different (though harmonious), and rarely will the rhythms line up. (To illustrate this basic concept, imagine that the two are simply singing "Row, Row, Row Your Boat". As one sings "Row, row, row your boat", the other is simultaneously singing "Gently down the stream". The pitches and rhythms of the lines are different, but they meld together well.)

Sometimes, two parts may mimic one another exactly (e.g. basses might sing a part that had been sung by sopranos a few bars back, albeit down an octave or two).

One effective implementation of imitative polyphony involves beginning a new "movement" of a piece with one vocal part while the other parts wrap up the previous "movement".

This holds the attention of the listener in much the same way that a soap opera holds the attention of the viewer: New plot lines are introduced before old ones are wrapped up.

A good example of a song that makes extensive use of imitative polyphony is Sicut Cervus, a motet by Palestrina.

Acknowledging that my own definition is specific to choral music, and is thus incomplete, I've also included two other definitions I've googled up.

imitative polyphony. A *style of *polyphonic writing where each of the *melodies mimics at least the beginning few *notes of an introductory melody. Historically, imitative polyphony is used for only brief passages of a *composition to add variety and help develop a musical idea. Some compositions, such as *fugues, are based entirely on imitative polyphony.

"In imitative polyphony, the same or nearly the same material is played by the successive voices, which come in one after the other, "imitating" their predecessors."

- http://www.stetson.edu/departments/human/handbook/PDF/Form.pdf