Sump meaning : A hollow or pit into which liquid drains, such as a cesspool, cesspit or sink. The lowest part of a mineshaft into which water drains. A completely flooded cave passage, sometimes passable by diving. The crankcase or oil reservoir of an internal combustion engine. The pit at the lowest point in a circulating or drainage system (FM 55-501). An intentional depression around a drain or scupper that promotes drainage. Of a cave passage, to end in a sump, or to fill completely with water on occasion.