Is There Any Such Thing as Absolute Synonyms?

A Nebi is someone who knows what the rest of mankind does not know. And Ras'ul is someone who has been sent with a message and he then engages in that message. The vast majority of scholars of our tradition, they said that there is a difference between a Nebi and Ras'ul. But whenever we have two words in the same language amongst the same group of people, the reality is that those two words never mean the same thing.