/ Dictionary / Index O open cluster: Meaning and Definition of Find definitions for: o'pen clus'ter Pronunciation: [key] — Astron. Astron. a comparatively young, irregularly shaped group of stars, often numbering up to several hundred, and held together by mutual gravitation; usually found along the central plane of the Milky Way and other galaxies. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. open classroom open communion Related Content Daily Word Quiz: arboreal Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language