numeral, pronoun, adverb, conjunction, measure word, noun, time word, expression, verb, idiom, proper name, adjective, particle, directional, punctuation, modal particle, preposition · the number one, single, or used as an indefinite article

我买了一个苹果。

adverb, numeral · indicating that one action follows immediately after another

他一到家就开始做饭。

numeral, adverb, noun, verb, measure word · entire, all, or throughout

他弄了一身泥。

numeral · the 'one' radical in Chinese characters (Kangxi radical 1)

老师告诉我们，这个汉字的部首是“一”。