From Wikipedia:
A slush fund, also known as a black fund, is any fund or account maintained for corrupt or illegal purposes, especially in the political sphere.[1] Such funds may be kept hidden and maintained separately from other funds that are used for legitimate purposes. They may be employed by government or corporate officials as part of efforts 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.[2] The funds themselves may not be kept secret but the source of the funds or how they were acquired or for what purposes they are used may be hidden. Use of slush funds to influence government activities may be viewed as subversive of the democratic process.