mass has 24 distinct synonyms spread across 4 of its 11 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 property of a body that causes it to have weight in a gravitational field
an ill-structured collection of similar things (objects or people)
(Roman Catholic Church and Protestant Churches) the celebration of the Eucharist
the common people generally
“separate the warriors from the mass”
the property of something that is great in magnitude
“it is cheaper to buy it in bulk”
a musical setting for a Mass
“they played a Mass composed by Beethoven”
a sequence of prayers constituting the Christian Eucharistic rite
“the priest said Mass”
formed of separate units gathered into a mass or whole
“aggregate expenses include expenses of all divisions combined for the entire year”
Synonyms, antonyms and the broader and narrower relations for mass come from Princeton University’s WordNet. Full sources and licences.