h has 7 distinct synonyms spread across 4 of its 5 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 nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe
a unit of inductance in which an induced electromotive force of one volt is produced when the current is varied at the rate of one ampere per second
the constant of proportionality relating the energy of a photon to its frequency; approximately 6.626 x 10^-34 joule-second
(thermodynamics) a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure
“enthalpy is the amount of energy in a system capable of doing mechanical work”
Synonyms, antonyms and the broader and narrower relations for h come from Princeton University’s WordNet. Full sources and licences.