QUESTION:
Please define the term 'cluster'

ANSWER from Joe Pesce on 20 March, 1996:
  A cluster, of course, is a collection or group of "something".  In 
 astronomy we have several types of clusters - star clusters and
 galaxy clusters.  

  Star clusters themselves come in two types:  open star clusters and globular
 star clusters.  Open star clusters are groupings of tens to thousands
 of stars and are found _within_ our Galaxy, the Milky Way, and other
 galaxies.  The Pleiades, or 7 sisters, is an example.  Go out
 at night and take a look with your naked eye, or binoculars (can
 you see seven stars? can you see more than seven stars?) - this 
 is a physical grouping of relatively young stars.  

  Globular star clusters are larger groups of thousands to millions
 of stars, all bound gravitationally into a ball-like structure
 (hence the name "globular").  These star clusters orbit our
 galaxy and others.

  Galaxy clusters, on the other hand, are groupings of galaxies.
 There are many different types, with tens to thousands of galaxies
 in each.  Our Galaxy, is the member of a small cluster or group
 of galaxies, all orbiting each other.

  In any case, whether stars or galaxies, all of the
 objects in clusters are bound gravitationally - which is what makes 
 the cluster a real physical object rather than just a projection of 
 lots of stars or galaxies, which just appear to be related (i.e. as is the
 case with constellations).