There are many broods of periodical cicadas. They are named with Roman Numerals (Brood X = Brood 10) based on what year they emerge.

For example, Brood X emerged in 2004, 1987, 1970, 1953 etc. It will emerge again in 2021 and every 17 years after that.

Brood XXIII (23) consists of 13-year cicadas. It emerged in 2002, 1989,1976, 1963, 1950, etc. It will emerge again in 2015.

An unusual feature of periodical cicadas is that each Brood consists of several species (3 or 4) that all emerge together at the same time and in the same place multiple species.