From WordNet (r) 3.0 (2006):
cluster
n 1: a grouping of a number of similar things; "a bunch of
trees"; "a cluster of admirers" [syn: bunch, clump,
cluster, clustering]
v 1: come together as in a cluster or flock; "The poets
constellate in this town every summer" [syn: cluster,
constellate, flock, clump]
2: gather or cause to gather into a cluster; "She bunched her
fingers into a fist" [syn: bunch, bunch up, bundle,
cluster, clump]