A guy asked me if my amp had a transistor, I just answered yes, don't know why:P

But what is it?
no no no, a transistor is slang for a transvestite sister.

in all seriousness, though, its the component that is used instead of tubes in a solid state amp.
Rig Winter 2017:

Fender Jazzmaster/Yamaha SG1000
Boss TU-3, DS-2, CS-3, EHX small stone, Danelectro delay
Laney VC30-112 with G12H30 speaker, or Session Rockette 30 for smaller gigs
Elixir Nanoweb 11-49 strings, Dunlop Jazz III XL picks
Shure SM57 mic in front of the amp