(1) A term used in the grammatical classification of words to refer to one of two postulated major word-classes in language, the other being invariable. Variable words are said to be those which express grammatical relationships through a change of form, e.g. boy / boys, walk / walking, nice / nicer. Invariable (or invariant) words are unchanging, whatever their distribution, e.g. in, on, and.
(2) The term has been introduced into sociolinguistics by the American linguist William Labov (b.1927) to refer to the units in a language which are most subject to social or stylistic variation, and thus most susceptible to change in the long term. Sets of phonological, grammatical and lexical variables are described quantitatively with reference to such factors as social class, age and sex, and the results of this co-variation are stated in the form of variable rules. Variable rules are generative rules which have been modified so as to specify the socio-regional conditions under which they apply. The notion has been developed primarily in relation to hypotheses concerning the relationship between social variation and linguistic evolution.
(3) The term is also widely used in grammar and semantics in its general sense of a symbol which may assume any of a set of values. For example, a category variable (e.g. X) stands for any level of bar projection of X (e.g. Xn stands for X0?, X?, X?). Pro-forms are often analysed semantically as variables, especially when they are bound by quantificational antecedents. In government-binding theory, the term refers to an A-bar-bound trace.
（1）語法有時假設語言的詞分為兩大類，即變形詞和不變形詞。變形詞是通過改變形式表達語法關系的詞，如boy / boys“男孩”（單數/復數），walk / walking“走”（原形/-ing形式），nice / nicer“好”（原級/比較級）。不變形詞不管分布如何都不改變形式，如in“在…里”，on“在…上”，and“和”。