portal has 4 distinct synonyms spread across 2 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 grand and imposing entrance (often extended metaphorically)
“the portals of the cathedral”
a site that the owner positions as an entrance to other sites on the internet
“a portal typically has search engines and free email and chat rooms etc.”
a short vein that carries blood into the liver
Synonyms, antonyms and the broader and narrower relations for portal come from Princeton University’s WordNet. Full sources and licences.