A concept album is an album tied together with a single concept. It can be anything from a linear coherent story such as Tommy, to something more abstract like Dark Side of the Moon. I have to admit, I didn't see a coherent concept tying DSOTM together until recently (and by recently, I mean less than 22 years ago). It's 'various aspects of life':
Speak to Me -- communication (everything run together at once being their sardonic statement on the subject)
Time -- time
On the Run -- travel
Great Gig in the Sky -- religion
Any Colour You Like -- vision
Us & Them -- conflict/war
Money -- money
etc.
Seems pretty clear to me now, in hindsight.
Mine:
Planet P Project, Pink World
Jeff Wayne, War of the Worlds
Rick Wakeman, The Myths and Legends of King Arthur and the Knights of the Round Table
Alan Parsons Project, I, Robot