Spillage is liquid (or small loose stuff like grain or powder) that has come out of its container by accident, and also the act or the fact of that happening. The everyday word is 'spill' — you say 'there's a spill in aisle four'. 'Spillage' is the same idea in the register of rules, forms and warning signs: supermarket staff notices, cleaning instructions, insurance documents, safety reports. That is the useful thing to know about it — the meaning is ordinary but the word is not, so using it in casual talk sounds oddly official, which some people do on purpose as a joke. It works both uncountably ('clean up any spillage immediately', 'the mat prevents spillage') and countably ('two spillages were reported last month'), with the uncountable use more common. It is used for the tiny and the huge alike: spillage from a coffee cup, and oil spillage from a tanker — though for the very large events people usually say 'oil spill'.