clus·ter 
(klŭstər)
(klŭstər)n.
1. A group of the same or similar elements gathered or occurring closely together; a bunch: "She held out her hand, a small tight cluster of fingers" (Anne Tyler).
2. Linguistics Two or more successive consonants in a word, as cl and st in the word cluster.
3. A group of academic courses in a related area.
v. clus·tered, clus·ter·ing, clus·ters
v.intr.
To gather or grow into bunches.
v.tr.
To cause to grow or form into bunches.
[Middle English, from Old English clyster.]