Antonyms of adj synonymous

1 sense of synonymous

Sense 1
synonymous (vs. antonymous) -- ((of words) meaning the same or nearly the same)

antonymous (vs. synonymous) -- (of words: having opposite meanings)
        => complementary -- (of words or propositions so related that each is the negation of the other; "`male' and `female' are complementary terms")
        => contradictory -- (of words or propositions so related that both cannot be true and both cannot be false; "`perfect' and `imperfect' are contradictory terms")
        => contrary -- (of words or propositions so related that both cannot be true but both may be false; "`hot' and `cold' are contrary terms")
        => contrastive, incompatible -- (of words so related that one contrasts with the other; "`rich' and `hard-up' are contrastive terms")
        => converse -- (of words so related that one reverses the relation denoted by the other; "`parental' and `filial' are converse terms")

Similarity of adj synonymous

1 sense of synonymous

Sense 1
synonymous (vs. antonymous) -- ((of words) meaning the same or nearly the same)
       => similar -- ((of words) expressing closely related meanings)
       => substitutable -- ((of words) interchangeable in a given context without changing the import of the expression)