A definition from communitybuilding.us:
"Social capital is the value of social relations between persons, colloquially referred to as “connections”. Like economic capital, social capital is a store of power that can be called upon to achieve certain ends and which operates according to certain local rules of exchange."
Definiations from socialcapitalresearch.com
"the value of social networks, bonding similar people and bridging between diverse people, with norms of reciprocity"
"the goodwill available to individuals or groups. Its source lies in the structure and content of the actor’s social relations. Its effects flow from the information, influence, and solidarity it makes available to the actor"
"friends, colleagues, and more general contacts through whom you receive opportunities to use your financial and human capital'
Many more definitons of social capital can be found at http://www.socialcapitalresearch.com/literature/definition.html
Wikipedia entry: social_capital