portion has 20 distinct synonyms spread across 8 of its 8 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.
something determined in relation to something that includes it
“he wanted to feel a part of something bigger than himself”
assets belonging to or due to or contributed by an individual person or group
“he wanted his share in cash”
your overall circumstances or condition in life (including everything that happens to you)
“whatever my fortune may be”
an individual quantity of food or drink taken as part of a meal
“the helpings were all small”
Synonyms, antonyms and the broader and narrower relations for portion come from Princeton University’s WordNet. Full sources and licences.