'白天' literally combines 'bái' (white) and 'tiān' (sky/heaven), evoking the bright, sunlit period of the day—essentially 'daytime'. Unlike '日间' (a more formal or literary term) or '白天里' (which adds a locative nuance), '白天' is neutral, widely used in speech and writing, and refers specifically to the hours between sunrise and sunset when natural light predominates.

This noun functions as a time noun: it can serve as a subject ('白天很热'), object ('我只在白天工作'), or adverbial modifier ('白天去公园'). It contrasts directly with '晚上' (evening/night) and is often paired with it in daily expressions like '白天和晚上'. While it may occasionally appear in idioms or fixed phrases, it rarely takes measure words and almost never appears without its core meaning of daylight hours.