Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
synōnymum, i, n., = συνώνυμον, a word having the same meaning with another, a synonym, Front. Eloqu. p. 237; Prisc. 579 P.; Serv. Verg. A. 2, 128. († synŏphītes, ae, m., a read. in Plin. 37, 10, 59, § 162 for synnephitis.)