warrant has 10 distinct synonyms spread across 4 of its 6 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a writ from a court commanding police to perform specified acts
a type of security issued by a corporation (usually together with a bond or preferred stock) that gives the holder the right to purchase a certain amount of common stock at a stated price
“as a sweetener they offered warrants along with the fixed-income securities”
formal and explicit approval
“a Democrat usually gets the union's endorsement”
provide adequate grounds to justify (a certain course of action)
“The emergency does not warrant all of us buying guns”
Synonyms, antonyms and the broader and narrower relations for warrant come from Princeton University’s WordNet. Full sources and licences.