An imperative is a grammatical mood that forms a command or request, used to demand or require that an action be performed. It is usually found only in the present tense, second person. Imperatives are often expressed using special conjugated verb forms, and like other finite verb forms, they often inflect for person and number. Second-person imperatives, used for ordering or requesting performance directly from the person being addressed, are most common, but some languages also have imperative forms for the first and third persons. Imperatives are used principally for ordering, requesting, or advising the listener to do (or not to do) something, and they are also often used for giving instructions as to how to perform a task. Imperatives can sometimes be seen on signs giving orders or warnings, or substituting one directive for another. Imperatives can also be used for speech acts whose function is not primarily to make an order or request, but to give an invitation, give permission, express a wish, make an apology, and so on.

In addition to being a grammatical mood, "imperative" can also be an adjective meaning "completely necessary" or "very important," but also "commanding". It can also be a noun meaning "a necessity" or "something that is not avoidable". When used in grammar, "imperative" refers to the imperative mood, which is one of the four main verb moods in the English language. The imperative mood indicates the state of commanding, and it is used to give commands, orders, instructions, advice, suggestions, and requests. Imperatives can be positive or negative, and they can be made more polite or formal by using the emphatic do auxiliary.