A grouping of a number of similar things.

(demographics) The grouping of a population based on ethnicity, economics or religion.

(computing) The undesirable contiguous grouping of elements in a hash table.

(writing) A prewriting technique consisting of writing ideas down on a sheet of paper around a central idea within a circle, with the related ideas radially joined to the circle using rays.

present participle and gerund of cluster

Forming a cluster.