There are two definitions of the word "dwija" that I know.
- Men of Brahmin , Kshatriya and Vaishya caste are dwija.
- Men who become alive again after dying once are called dwija. This is the reason why seers before being initiated into any sect have to do their "Pinda- Daan" or "Shraadh" which indicates that they have died .
I think the concept of dying here means men who have killed all their worldly desires or may be they really die by removing their soul from their body and then re-enter their body.(not sure about this).
Where is the second definition of dwija written in Hindu scriptures ?