unmarried

Compared meaning: not married (adjective).

比较义项：未婚，没有配偶（形容词）。

• single

  Common, everyday word for not being married; can also simply mean not currently in a romantic relationship, and is used freely in casual conversation.

  常见的日常用词，表示未婚；也可以单纯指没有恋爱对象，常用于日常口语交流中。

  My brother is thirty-five and still single, though he says he's happy that way.

  我哥哥三十五岁了还是单身，不过他说他这样很开心。

• unmarried 本词

  Formal, literal term used mainly in official, legal or written contexts to state marital status precisely, without implying anything about dating or relationships.

  较为正式、字面化的用词，主要用于官方、法律或书面语境，客观说明婚姻状态，不涉及恋爱关系。

  The form asks whether you are married or unmarried before listing your dependents.

  表格要求先说明你是已婚还是未婚，然后再填写受抚养人信息。

Single and unmarried both mean not married. Single is common and casual, and can also simply mean not in a romantic relationship. Unmarried is formal, used mainly in official or written contexts to state marital status plainly, without suggesting anything about dating.

这两个词在“未婚”这个意义上很接近，但语体和用法不同：single是日常口语用词，也可以泛指没有恋爱对象；unmarried更正式，多用于官方或书面语境，只客观说明婚姻状况。这里不比较single的“单一的、一个的”等其他意义。