I would translate 'contrato social' literally into 'social contract'. It is the theory that the governed are implicitly entering into a contract to follow the state's law when they are born.
I believe Articles of agreement & Partnership agreement are terms used for explicit contracts.
You can compare two wiki pages and see they talk of the same concept:
http://pt.wikipedia.org/wiki/Contrato_social
http://en.wikipedia.org/wiki/Social_contract