atm has 10 distinct synonyms spread across 3 of its 3 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 unit of pressure: the pressure that will support a column of mercury 760 mm high at sea level and 0 degrees centigrade
a means of digital communications that is capable of very high speeds; suitable for transmission of images or voice or video as well as data
“ATM is used for both LAN and WAN”
an unattended machine (outside some banks) that dispenses money when a personal coded card is used
Synonyms, antonyms and the broader and narrower relations for atm come from Princeton University’s WordNet. Full sources and licences.