Noun

1. The action of the verb to cluster.
2. A grouping of a number of similar things.
3. The grouping of a population based on ethnicity, economics or religion.
4. The undesirable, contiguous grouping of elements in a hash table.
5. 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.