What is Voluntary Sector?

Legal Definition
The voluntary sector or community sector (also non-profit sector or "not-for-profit" sector) is the duty of social activity undertaken by organizations that are not-for-profit and non-governmental. This sector is also called the third sector, in contrast to the public sector and the private sector. Civic sector or social sector are other terms for the sector, emphasizing its relationship to civil society. Given the diversity of organizations that comprise the sector, Peter Frumkin prefers "non-profit and voluntary sector".
-- Wikipedia
Legal Definition
Part of a not-for profit segment of the economy where quasi public institutions are excluded. Such as hospitals.