What is "modal verb"

n. (context grammar English) A verb whose primary function is to express mood.

A modal verb (also modal, modal auxiliary verb, or modal auxiliary) is a type of verb that is used to indicate modality – that is: likelihood, ability, permission, and obligation. Examples include the English verbs can/could, may/might, must, will/would, and shall/should. In English and other Germanic languages, modal verbs are often distinguished as a class based on certain grammatical properties.