entropy has 4 distinct synonyms spread across 2 of its 2 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.
(communication theory) a numerical measure of the uncertainty of an outcome
“the signal contained thousands of bits of information”
(thermodynamics) a thermodynamic quantity representing the amount of energy in a system that is no longer available for doing mechanical work
“entropy increases as matter and energy in the universe degrade to an ultimate state of inert uniformity”
Synonyms, antonyms and the broader and narrower relations for entropy come from Princeton University’s WordNet. Full sources and licences.