pronoun · The third person or item in a list, sequence, or series.

在列出的三项提议中，第三者最难实现。

noun · A person who is romantically involved with someone already in a committed relationship, such as a husband or wife.

由于第三者的介入，他们的婚姻最终走向了破裂。

numeral · A person or entity besides the two primarily involved in a situation, dispute, or legal agreement; a disinterested party.

车主必须购买第三者责任险。