Cyclin

A family of proteins whose concentrations rise and fall in a cyclical pattern during the cell cycle, regulating progression through different phases by activating cyclin-dependent kinases (CDKs). Different cyclins control different cell cycle transitions.

Named 'cyclin' in the 1980s because these proteins were discovered to accumulate and then suddenly disappear in cycles that matched cell division timing. The name perfectly captured their oscillating behavior during the cell cycle.