twenty-four-hour clock

American  

[twen-tee-fawr-ou^uhr klok, ou-er, twuhn-tee-] / ˈtwɛn tiˈfɔrˌaʊər ˈklɒk, ˌaʊ ər, ˈtwʌn ti- /

Or 24-hour clock

noun

1. a system of time measurement in which hours in the day are counted off consecutively from 0 to 24, with midnight being either 00:00 or 24:00.

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Etymology

Origin of twenty-four-hour clock

First recorded in 1780–90