Wikipedia:

Cluster

Summary:

a small group or bunch of something. In science: Cluster (physics), a small group of atoms or molecules | Cluster chemistry, an array of bound atoms intermediate in character between a molecule and a solid | Cluster sampling, a sampling technique ...  (39 of 368 words, 27 senses)

Link:

http://en.wikipedia.org/wiki/Cluster  [cite]

Encarta:

cluster | clusters [plural] | clustered [past tense] | clustering [present participle] | clustered [adjective] | clustery [adjective]

Summary:

a small group of people or things that are closely packed together | a group of galaxies or stars that are gravitationally interacting in space and ...  (25 of 216 words, 10 senses, 1 etymology, pronunciation)

Link:

http://encarta.msn.com/dictionary 1861598073/definition.html  [cite]

Random House:

cluster

Summary:

a number of things of the same kind, growing or held together; a bunch. | a group of things or persons close together. | [United States Army] a small ...  (27 of 147 words, 8 senses, pronunciation)

Link:

http://dictionary.infoplease.com/cluster  [cite]

Wiktionary:

cluster | clusters [plural] | clustered [past tense] | clustering [present participle] | cluster analysis | cluster bomb | globular cluster | open cluster | star cluster

Summary:

A group or bunch of several discrete items that are close to each other. | [astronomy] A group of galaxies or stars that appear near each other. | [music] A chord of three or more notes. | [phonetics] A group of consonants. | [computing] A group of ...  (43 of 132 words, 9 senses, 2 images, 1 etymology, pronunciation)

Link:

http://en.wiktionary.org/wiki/cluster  [cite]

Merriam-Webster:

cluster | clustery

Summary:

a number of similar things that occur together: as | two or more consecutive consonants or vowels in a segment of speech | a group of buildings and ...  (26 of 123 words, 5 senses, 1 etymology, pronunciation)

Link:

http://www.merriam-webster.com/dictionary/cluster  [cite]

Britannica:

cluster

Summary:

Atoms and molecules are the smallest forms of matter typically encountered under normal conditions and are in that sense the basic building blocks of ...  (24 of 99 words)

Link:

http://www.britannica.com/EBchecked/topic/122615/cluster  [cite]

Columbia:

cluster

Summary:

in astronomy: see star cluster; galaxy.  (6 of 8 words)

Link:

http://www.infoplease.com/ce6/sci/A0911966.html  [cite]