Prolation describes the structure of rhythms on a small scale, usually used in the theory of medieval music. Prolation essentially corresponds to the modern conception of time signatures.
Prolation describes whether a whole note is equal to two half notes or three half notes. Minor prolation is where a whole note is equal to three half notes and major prolation is where a whole notes is equal to two half notes.
In medieval music, note values were commonly subdivided into groups of three, while in modern music they are usually subdivided into groups of two. Since medieval times music has shifted from mostly major prolation to predominately minor prolation.