modal verb
A helping verb (like can, must, or should) that expresses possibility, necessity, or permission rather than tense.
A modal verb (or modal auxiliary) is a helping verb — can, could, may, might, must, shall, should, will, would — that expresses possibility, necessity, permission, or obligation instead of marking a plain tense, and never changes form or takes "to" before the verb that follows it.
Examples
You must finish this by Friday.
She might arrive late.
See also