性(grammatical gender)在语法中是指一个名词或代词的类别，以及形容词、冠词或动词在与名词或代词搭配时发生的屈折变化。因此，性对于名词或代词来说是一种分类，而对于形容词、冠词或动词来说是一种屈折，两者的意义是不同的。不过在名词中，一些指人或生物的名词也可以看作有性的屈折，如德语中的der Student（大学生）－die Studentin（女大学生），der Löwe（雄狮）－die Löwin（雌狮），但也可以把这些屈折看作是不同的词汇。

不是所有的语言都有性这一语法范畴。有性的语言主要存在于印欧语系与闪含语系当中。

In linguistics, grammatical gender is a specific form of noun-class system in which the division of noun classes forms an agreement system with another aspect of the language, such as adjectives, articles, pronouns, or verbs. This system is used in approximately one quarter of the world's languages. In these languages, most or all nouns inherently carry one value of the grammatical category called gender; the values present in a given language (of which there are usually two or three) are called the genders of that language. According to one definition: "Genders are classes of nouns reflected in the behaviour of associated words."