Semitec 100kOhm NTC Thermistor. (104GT-2)
For the datasheet, see the ATC Datasheet.
See also: RepRap Wiki.
E3D HotEnd Compatibility:
If your v5 HotEnd was shipped with one of these thermistors, then it is compatible. This thermistor is not compatible with earlier revisions of the E3D HotEnd. See this blog post for more information.