My characteristically rambling answer to Charmian's question is, therefore, below ;-)
1) A community is a number of individuals connected by the spirit of belonging to something synergic, or (to steal a phrase) greater than the sum of it's parts. To me the word 'community' evokes the notion "we're stronger together."
2) My conceptual community has an established set of core values shared by all members and, of which, the maintenance is equally and autonomously the responsibility of each individual. So the community is, therefore, self governing.
3) There is an established culture of co-operation to resultant mutual benefit which allows for the creation of a creative space to cultivate and grow ideas.
The definition I have settled upon is 621 characters long. Interestingly, it happily suits 'virtual' and 'cyber' communities as well as neighbourhoods and, I would assert, is the foundation for the happiest families I know.
It is also, I have just realised, pretty damn close to the definition of Anarchy .... oh well ;-)
Think you can do better? Go tell her yours!