1. [n]  a grouping of a number of similar things; "a bunch of trees"; "a cluster of admirers"
2. [v]  gather or cause to gather into a cluster; "She bunched her fingers into a fist"; "The students bunched up at the registration desk"
3. [v]  come together as in a cluster or flock; "The poets constellate in this town every summer"

1. \Clus"ter\, n. [AS. cluster, clyster; cf. LG. kluster
   (also Sw. & Dan. klase a cluster of grapes, D. klissen to be
   entangled?.)]
   1. A number of things of the same kind growing together; a
      bunch.
            Her deeds were like great clusters of ripe grapes,
            Which load the bunches of the fruitful vine.
                                                  --Spenser.
   2. A number of similar things collected together or lying
      contiguous; a group; as, a cluster of islands. ``Cluster
      of provinces.'' --Motley.
   3. A number of individuals grouped together or collected in
      one place; a crowd; a mob.
            As bees . . . Pour forth their populous youth about
            the hive In clusters.                 --Milton.
            We loved him; but, like beasts And cowardly nobles,
            gave way unto your clusters, Who did hoot him out o'
            the city.                             --Shak.
   

2. \Clus"ter\, v. i. [imp. & p. p. {Clustered}; p. pr. &
   vb. n. {Clustering}.]
   To grow in clusters or assemble in groups; to gather or unite
   in a cluster or clusters.
         His sunny hair Cluster'd about his temples, like a
         god's.                                   --Tennyson.
         The princes of the country clustering together. --Foxe.
   

3. \Clus"ter\, v. t.
   To collect into a cluster or clusters; to gather into a bunch
   or close body.
         Not less the bee would range her cells, . . . The
         foxglove cluster dappled bells.          --Tennyson.
         Or from the forest falls the clustered snow. --Thomson.
   {Clustered column} (Arch.), a column which is composed, or
      appears to be composed, of several columns collected
      together.
   

An elementary unit of allocation of a disk made up of one or more physical blocks.

A file is made up of a whole number of possibly non-contiguous clusters. The cluster size is a tradeoff between space efficiency (the bigger is the cluster, the bigger is on the average the wasted space at the end of each file) and the length of the FAT.