slush fund

A slush fund (also black fund), colloquially, is an auxiliary monetary account or a reserve fund. However, in the context of corrupt dealings, such as those by governments or large corporations, a slush fund can have particular connotations of illegality, illegitimacy, or secrecy in regard to the use of this money and the means by which the funds were acquired. Funds are usually made to discreetly pay influential people in return for preferential treatment, advance information (for example, to acquire non-public information in financial transactions) or some other service.