This term has probably as many definitions as those willing to define it; for our purposes this one will have to do: the collectivity of all voluntary institutions—ranging from trade unions to professional organizations, from activist organizations dealing with the environment to organizations concerned with human rights—in a society that are constituted from outside the arenas of the family, the state, and the market place.
In a democracy, civil society is its basic foundation (to put it bluntly: no civil society, no democracy). There is a dialectical relationship between civil society and democracy, where one nourishes the other.[1]
[1]. Two introductory books worth looking at that explore this concept in its various manifestations is the anthology edited by Glasius, Lewis, and Seckinelgin (2004), and the Oxford Handbook of Civil Society.