空调(kōng tiáo) - air conditioning, air conditioner in Mandarin

空调 (kōngtiáo) is air conditioning, shortened from the fuller 空气调节 (kōngqì tiáojié), "air regulation". Almost nobody says the long form; 空调 is the word you will actually meet, in flats, offices, trains and shops alike.

It takes the appliance measure word 台 (tái): 一台空调 (yì tái kōngtiáo). Usefully, the same word covers both cooling and heating on a reversible unit, so 开空调 (kāi kōngtiáo) just means "turn the air conditioning on" without specifying which direction, and you often need 制冷 (zhìlěng), "cooling mode", or 制热 (zhìrè), "heating mode", to be precise.

空 (kōng) on its own means "empty" or "air/sky" depending on context, and 调 (tiáo) means "to adjust, to tune", the same character you get in 调整 (tiáozhěng), "to adjust". Neither half means "conditioning" by itself; the sense only lands once they are paired.