the first thing that popped into my head was 'potentiometer,' but I haven't a clue what kind of variable resistor I would need to look at for 120v 15A power.
edit - I guess I would want to look at something like this:
Amazon.ca
www.amazon.ca
No specific recommendations from me as the only ones I've used have been fairly large industrial units from the cold war era, but yeah, that kinda looks like what I'd be looking at for my own use.