A cluster is a small group or bunch of something.

In science:

• Cluster (physics), a small group of atoms or molecules
• In astrophysics:
• Star cluster, groups of stars which are gravitationally bound
• Galaxy groups and clusters, large gravitationally-bound objects
• Globular cluster
• Cluster mission, a European Space Agency mission to study the magnetosphere
• Cluster chemistry, an array of bound atoms intermediate in character between a molecule and a solid
• Cluster sampling
• Data clustering or Cluster analysis, a technique for statistical data analysis
• Consonant cluster, in linguistics

In biology and health sciences:

• In genetics and biotechnology, a cluster may refer to a group of closely gathered genes within a chromosome.
• Cancer cluster, in biomedicine, an occurrence of a greater-than-expected number of cancer cases
• Cluster headache
• Winter cluster, in beekeeping, a well-defined cluster of honey bees in cold temperatures
• Cluster (epidemiology) refers to a grouping of cases of disease, possibly an outbreak
• In genomics, a cluster often refers to a group of genes (or proteins, or metabolites) whose expression or concentration is similar across a range of conditions (see data clustering).

In computing:

• Cluster (computing), a group of loosely coupled computers that work together closely
• Cluster (file system), a group of disk sectors used in a File Allocation Table
• Comparison of cluster software

In economics:

• Business cluster, a geographic cluster of competitive businesses

In science fiction:

• Cluster (novels) - novels of Piers Anthony

In music:

• Tone cluster, in music
• Cluster (band), a German rock band
• Cluster (album), the 1971 eponymous album by the band Cluster
• Cluster, Ben Bu Sarkiyi Sana Yazdim album track by the singer Cem Adrian

Other

• A less common name for a bicycle cogset, either a freewheel, or cassette.
• Cluster bomb, air-dropped or ground launched shells
• Cluster diagram, general type of diagram, which represents some kind of cluster.