刻(kè) - quarter of an hour in Mandarin

刻 (kè) is a quarter of an hour, fifteen minutes, and it is how Mandarin says "quarter" on a clock face. 三点一刻 (sān diǎn yí kè) is "quarter past three", the quarter added on after the hour.

Going the other way, 差 (chà, "short of, lacking") pairs with 刻 (kè) to make "quarter to": 差一刻五点 (chà yí kè wǔ diǎn) is "quarter to five", literally "short a quarter till five o'clock".

On its own, away from a clock face, 一刻钟 (yí kè zhōng) is the standalone noun for "a quarter of an hour" - the natural way to ask someone to wait, or to say how long something took.

Do not confuse 刻 (kè, "quarter hour") with 课 (kè, "lesson, class") - identical pinyin and tone, completely different character. One belongs on a clock, the other on a timetable, and context sorts them out at a glance.