Stoker used the term in his novel of 1897, where he “translates” it to mean “undead.” In fact, he borrowed the term (and this connotation) from Emily Gerard’s The Land Beyond the Forest (1885), where she explicitly identifies it as Romanian for “vampire.” Before this, Wilhelm Schmidt also identified the term as meaning vampire, but without ever specifying the language.

Another source is Roumanian Superstitions (1861) by Heinrich von Wlislocki, which gives the term a richer meaning. A “nosferat” is both blood-drinker and incubus/succubus. Wlislocki’s nosferat drinks the blood of older people, while seeking to copulate with younger ones (especially newlyweds), and thus becomes the supposed cause of illegitimate children, impotence, and infertility.

The three candidate roots for the term are necurat (“unclean”, as in a possessing spirt), nesuferit (“insufferable”), and nefârtat (“enemy” or “the Devil”, lit. “unbrothered”).

According to Wikipedia, nesuferitu is the most phonetically satisfactory of the three, which makes the identification with vampire an extra-Romanian fabrication.