Valency refers to the capacity of a verb to take a specific number and type of arguments (noun phrase positions).
The terminology comes from chemistry, in which the valency of a chemical element is its capacity for combining with a fixed number of atoms of another element—for example, hydrogen can bond with only one other element, and is called monovalent.
Verbs can be divided into classes based on their valency (how many arguments or ‘valents’ they can take). In some languages, these classes may have distinctive morphosyntactic characteristics, such as unique case marking patterns, or restrictions on tense/aspect/modality marking.
Here are some examples of possible verb classes based on valency: