Definition - The number of subjects and objects that a verb can govern.

Example -

(1) intransitive verbs can only have one subject (e.g. I snore), thus have a valency of 1;
(2) transitive verbs can have a subject and a direct object (e.g. I eat spinach), and thus have a valency of 2;
(3) ditransitive verbs can have a subject, a direct object, and an indirect object (e.g. I gave him a drink); thus have a valency of 3.

Etymology -
The word ultimately derives from the Latin valentia, strength or capacity (from valere, be strong).
Note: The linguistics' sense of the term was probably coined on analogy with its chemistry sense, the combining power of an element; Linguistic valency thus denoting the combining power of the verb.

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