WordNet records no synonym and no antonym for ream. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
squeeze the juice out (of a fruit) with a reamer
“ream oranges”
remove by making a hole or by boring
“the dentist reamed out the debris in the course of the root canal treatment”
a large quantity of written matter
“he wrote reams and reams”
a quantity of paper; 480 or 500 sheets; one ream equals 20 quires
Synonyms, antonyms and the broader and narrower relations for ream come from Princeton University’s WordNet. Full sources and licences.