A slush fund, colloquially, is an auxiliary monetary account or a reserve fund. However, in the context of corrupt (including but not limited to) political dealings by governments, large corporations or other bodies and individuals, a slush fund can have particular elements of illegality, illegitimacy, or secrecy in regard to the use of this money and the means by which the funds were acquired.
Additional meaning of Slush Fund:
The term slush fund is used in accounting to describe a general ledger account in which all manner of transactions can be posted to commingled funds and "loose" monies by debits' and credits' cancelling each other out.