Traianus

Latin

Probably from Trā̆ius +‎ -ānus/-iānus: Trajan's paternal grandmother is assumed to have been from the gens Traia. The source of the nomen Trā̆ius is uncertain, but it may be of Oscan origin.

(Classical Latin) IPA^(key): /traːˈi̯aː.nus/, [t̪räːˈi̯äːnʊs̠] or IPA^(key): /trai̯ˈi̯aː.nus/, [t̪räi̯ˈi̯äːnʊs̠]
(modern Italianate Ecclesiastical) IPA^(key): /traˈja.nus/, [t̪räˈjäːnus]
Traia- scans as two heavy syllables. Given the uncertain etymology, it is unclear whether the first syllable contained a short vowel followed by double /jj/ or a long vowel followed by single /j/. (Many dictionaries mark the vowel as long in either of these contexts.) The pronunciation with short /a/ and double /jj/ would be more consistent with the usual pronunciation of intervocalic -i- as /jj/ in unprefixed Latin words (such as aio, maior, Maius, eius, Troia). The spelling "Traiianus" is attested on some inscriptions, such as the Tabula Banasitana,^[1] although this does not necessarily reveal the pronunciation.

Trā̆iānus m (genitive Trā̆iānī); second declension

^ Euzennat, Maurice, Seston, William (1980) “Un dossier de la chancellerie romaine : la Tabula Banasitana. Etude de diplomatique”, in Publications de l'École Française de Rome‎^[1], volume 43, pages 87-89, 95