The Collaborative International Dictionary of English v.0.48:

Cluster \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.
   [1913 Webster]

         His sunny hair
         Cluster'd about his temples, like a god's. --Tennyson.
   [1913 Webster]

         The princes of the country clustering together. --Foxe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

clustering
    n 1: a grouping of a number of similar things; "a bunch of
         trees"; "a cluster of admirers" [syn: bunch, clump,
         cluster, clustering]

The Free On-line Dictionary of Computing (30 December 2018):

cluster
clustering

   1.  Multiple servers providing the same
   service.  The term may imply resilience to failure and/or
   some kind of load balancing between the servers.  Compare
   RAIS.

   2.  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.

   (1996-11-04)