A Euclidean rhythm distributes a number of hits as evenly as possible over a number of steps. For example, three hits in eight steps give a pattern that appears in many world music traditions.
The method comes from Godfried Toussaint, who showed in 2005 that the Euclidean algorithm generates many traditional rhythms. Euclidean sequencers let you set steps and hits, and often a rotation, to create patterns quickly.