Family Characidae (tetras)
Subfamily Serrasalminae (piranhas and their relatives)
Piranhas include anytihng within the genera serrasalmus, pygocentrus, pygopristis, pristobrycon, and depending on who you are talking to, catoprion anything else in the serrasalminae subfamily that does not fall into one of those genera are either a silver dollar or pacu.
if you going to call them anything call them tetra's atleast that is scientificaly correct (same could be said for piranhas i geuss)