The amount of people or things that can fit inside a room, or a gathering of people together in a room.
A compound of 'room' (from Old English 'rum' meaning space) and the suffix '-ful' (meaning as much as can fill), creating 'as much as a room can hold.'
The word 'roomful' is a measurement unit created by regular people—like 'handful' or 'mouthful'—and it's charmingly imprecise because every room is different, yet we all understand it perfectly.
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