Prime interval
A prime interval is a musical interval which as a ratio of frequencies is a prime number; that is, a number such as 2, 3, 5, 7, ... which is divisible only by itself and 1. Any musical interval in the p-limit can be expressed in terms of a product of prime numbers less than or equal to p.
Intervals such as 5/3 are not considered prime intervals; the prime intervals are 2/1, 3/1, 5/1, 7/1, etc.