Definition of must :
1. An elephant in must.
2. Being in a condition of dangerous frenzy, usually connected with sexual excitement; - said of adult male elephants which become so at irregular intervals.
4. The condition of frenzy.
5. The expressed juice of the grape, or other fruit, before fermentation.
6. To be morally required; to be necessary or essential to a certain quality, character, end, or result; as, he must reconsider the matter; he must have been insane.
7. To be obliged; to be necessitated; - expressing either physical or moral necessity; as, a man must eat for nourishment; we must submit to the laws.
8. To make musty; to become musty.
obligation (part of speech: noun)
requirement (part of speech: noun)
required (part of speech: adjective)
inescapable, mandatory, compulsory, pressing, proviso, necessary, indispensable, involuntary, requisite, constraining, inevitable, obligatory, required, imperative, prerequisite, stipulated, essential, binding, inexorable