plaster has 10 distinct synonyms spread across 8 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.
a mixture of lime or gypsum with sand and water; hardens into a smooth solid; used to cover walls and ceilings
any of several gypsum cements; a white powder (a form of calcium sulphate) that forms a paste when mixed with water and hardens into a solid; used in making molds and sculptures and casts for broken limbs
a medical dressing consisting of a soft heated mass of meal or clay that is spread on a cloth and applied to the skin to treat inflamed areas or improve circulation etc.
a surface of hardened plaster (as on a wall or ceiling)
“there were cracks in the plaster”
adhesive tape used in dressing wounds
Synonyms, antonyms and the broader and narrower relations for plaster come from Princeton University’s WordNet. Full sources and licences.