penetrable has no recorded synonym. 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. Its one recorded opposite is impenetrable.
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.
admitting of penetration or passage into or through
“a penetrable wall”
1 of the 2 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for penetrable.
Synonyms, antonyms and the broader and narrower relations for penetrable come from Princeton University’s WordNet. Full sources and licences.