1) A small opening designed into a stove through which pressurized fuel passes so it can mix with air and burn efficiently. Jets were slowly developed over centuries by legions of engineers patiently working through a deliberate process of trial and error just so you could have a backpacking stove, you selfish, thoughtless bastard.
2) An opening in the burner of a stove where the flame is produced. Also, the flame itself. (Ffffft!)
3) Still not sure? It's like a snot rocket, but hotter, though generally should not be fitted nasally.
See tabs at the top for definitions and books.
Have anything worth adding? Then try sosayseff+eff@gmail.com
Me? Looking for my pipe cleaner.