A cyclin is any number of proteins that help regulate the cell cycle and whose abundance rises and falls rhythmically during the cell cycle. Most cyclins appear abruptly and disappear a short time later. Each cyclin appears at it's characteristic time in the cell cycle because it's transcription is linked to the cell cycle via a previously expressed cyclin. Eukaryotes can have more than one cyclin for a specific stage in the cell cycle; for example, cycling Cln1, Cln2 and Cln3 all function in the G1 stage in budding yeast. Another example is cyclin B which is most abundant when cells enter mitosis. The exit of mitosis requires the destruction of cyclin B.
Cyclins can also form cyclin-CDK complexes by binding cyclin subunits with cyclin-dependent protein kinases. All eukaryotes regulate the progression through the cell cycle by means of cyclin-CDK complexes.