m has 18 distinct synonyms spread across 6 of its 7 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.
the basic unit of length adopted under the Systeme International d'Unites (approximately 1.094 yards)
concentration measured by the number of moles of solute per liter of solution
the cardinal number that is the product of 10 and 100
a unit of information equal to 1000 kilobytes or 10^6 (1,000,000) bytes
a unit of information equal to 1024 kibibytes or 2^20 (1,048,576) bytes
the 13th letter of the Roman alphabet
denoting a quantity consisting of 1,000 items or units
Synonyms, antonyms and the broader and narrower relations for m come from Princeton University’s WordNet. Full sources and licences.