一半 (yí bàn) literally means 'one half' — combining 一 (yī), the numeral 'one', and 半 (bàn), meaning 'half'. It functions as a numeral expressing exactly 50% of a whole quantity, whether countable or uncountable. Unlike English, it’s not used alone as a pronoun (e.g., you wouldn’t say 'I’ll take 一半' without specifying what is being halved); it usually follows a measure word or noun (e.g., 一半苹果, 一半时间) or appears after a noun with 的 (e.g., 这本书的一半). It commonly appears in contexts involving division, sharing, estimation, or proportion.

The tone of 一 changes to yí before fourth-tone syllables like bàn — a standard tone sandhi rule. While 一半 can refer to physical halves (like cutting an apple), it’s equally frequent in abstract contexts: time, effort, responsibility, or probability. It does not imply approximation — for that, use 差不多 or 大概. In formal writing or speech, it may be replaced by 二分之一, but 一半 is far more common in daily spoken and written Chinese.