n. (pl. -fuls)
1. the amount held by a cup, esp. US a half-pint or 8-ounce measure in cookery.
2 a cup full of a substance (drank a cupful of water).
Usage:
A cupful is a measure, and so three cupfuls is a quantity regarded in terms of a cup; three cups full denotes the actual cups, as in three cups full of water. Sense 2 is an intermediate use.