The universal term for the part that provides power (Mod) to the part that holds your liquid and heating element (tank and coil), the name originates from the very first power assemblies being literally Modified flashlights. These days countless companies specifically manufacture battery assemblies as personal diffuesers, nonetheless the term has stuck.
You'll generally also want something to hold your liquid and diffuse it, in addition to one of these fine parts. Check out our excellent selection of Regular Tanks or Subohm Tanks. Hobbyists rocking the mech mods should peruse our exquisite RDAs and RTAs.