20th-century composer Elliott Carter died last month. RIP. He’s the inventor of the term metric modulation; I did a term paper on it in college. Sometimes called “tempo modulation”, Nicolas Slonimsky describes it like this:
A technique in which a rhythmic pattern is superposed on another, heterometrically, and then supersedes it and becomes the basic meter. Usually, such time signatures are mutually prime, e.g., 4/4 and 3/8, and so have no common divisors. Thus the change of the basic meter decisively alters the numerical content of the beat, but the minimal denominator (1/8 when 4/4 changes to 3/8; 1/16 when, e.g., 5/8 changes to 7/16, etc.) remains constant.
Here’s an example of simple metric modulation found in a piece by J.S. Bach: