in grammar, a category that reflects the speaker's view of the ontological character of an event. This character may be, for example, real or unreal, certain or possible, wished or demanded. Mood is often marked by special verb forms, or inflections, but it is sometimes expressed by a single word or a phrase.

Languages frequently distinguish grammatically three moods: the indicative, the imperative, and the subjunctive. The indicative is generally used for factual or neutral situations, as in English “John did his work” and Spanish “Juan hizo su trabajo.” The imperative conveys commands or requests—for example, “Do your work.” It is distinguished by the absence of an explicit subject, the implied subject being “you.” The Spanish imperative, which also possesses an implied subject, assumes a distinct verbal form, as in “Haga su trabajo.” The functions of the subjunctive mood vary widely across languages. Some notions often expressed by the subjunctive are doubt, possibility, necessity, desire, and future time. The English subjunctive is fairly limited in its use. Usually, it is found only in formal styles, such as the sentence “It is necessary that he be ready on time.” More often, subjunctive meanings are expressed by modal auxiliary verbs, such as can, must, or may, as in “He must be ready on time.”

Other moods sometimes grammaticalized in languages include conditional, hortative (urging), dubitative (doubting), optative (wishing), hypothetical, and potential.

in logic, the classification of categorical syllogisms according to the quantity (universal or particular) and quality (affirmative or negative) of their constituent propositions. There are four forms of propositions: A (universal affirmative), E (universal negative), I (particular affirmative), and O (particular negative). Because each syllogism has three propositions and each proposition may take four different forms, there are 64 different patterns (moods) of syllogisms. Twenty-four of the 64 possible moods are valid, though only 19 were traditionally accepted as valid. Various mnemonic terms are employed to label these moods. The vowels of these terms represent the forms of propositions in the syllogism. For example, “Felapton” is the mnemonic term to signify the mood in which the major premise (the premise containing the predicate of the conclusion) of the syllogism is an E proposition, the minor premise (the premise containing the subject of the conclusion) is an A, and the conclusion is an O.