定义（Definition）是透过列出一个事件或者一个对象的基本属性来描述或规范一个词或一个概念的意义；被定义的事物或者对象叫做被定义项，其定义叫做定义项。

例如“一个单身汉是一个未婚男子”这个定义中“单身汉”是被定义项，“未婚男子”是定义项。定义中的“一个”和“是”均可以使用符号取代，比如使用：=或 =_df这个符号，上面这个定义可以转写为：“单身汉：=未婚男子”或“单身汉=_df未婚男子”。一般来说一个定义像上面这个例子一样往往是表达被定义项与定义项之间的等同的句子。

A definition is a statement of the meaning of a term (a word, phrase, or other set of symbols). Definitions can be classified into two large categories, intensional definitions (which try to give the essence of a term) and extensional definitions (which list every single object that a term describes). A term may have many different senses and multiple meanings, and thus require multiple definitions.