Jim - my definition (and here I open myself up to a possible hail of dissent), here in the UK, would be an event usually held in the bar of a pub, with the pub's customers on hand to watch and listen. An event where other customers come along and sing something, perform something, read something, tell jokes, do any kind of act that they choose. There's usually a performer/compere - paid or unpaid by the pub - who takes down names and introduces the acts.

No club membership - no entrance fees - no performing fees - no proscriptive genre - no prizes. Just step up with your performing frog and do your act. A chance for entertainers, in the broadest sense of the word, to try their act out on the public. Open mic = open to anyone and anything.