flare has 14 distinct synonyms spread across 8 of its 15 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 burst of light used to communicate or illuminate
reddening of the skin spreading outward from a focus of infection or irritation
a sudden recurrence or worsening of symptoms
“a colitis flare”
a sudden eruption of intense high-energy radiation from the sun's surface; associated with sunspots and radio interference
am unwanted reflection in an optical system (or the fogging of an image that is caused by such a reflection)
a sudden outburst of emotion
“she felt a flare of delight”
a short forward pass to a back who is running toward the sidelines
“he threw a flare to the fullback who was tackled for a loss”
(baseball) a fly ball hit a short distance into the outfield
Synonyms, antonyms and the broader and narrower relations for flare come from Princeton University’s WordNet. Full sources and licences.