Slush Fund
An informal budget reserve for unplanned needs, existing in the gap between official accounting and operational reality. Every experienced manager has one; none admit it in writing.
Heard in the wild
“The 'miscellaneous supplies' line is obviously a slush fund, and it saved Q3.”
Tone check: Danger
Ubiquitous and unwise to document. If yours is discovered, its name becomes 'budget discipline finding.'