But I like to pick flowers!
I'll try to guess...
Powerhead is a fancy version of
motorhead, which in turn is an archaic term used for the business part of a motorized device, like an engine, vacuum cleaner, etc. The powerhead is where energy (from electricity, burning fuel...) is translated into rotary motion.
It used to be that small
submersible pump motors had all sorts of add-ons to go with them (piping, filtration canisters, --certainly the impeller, etc ). Thus it sometimes made for convenience to refer to the set as
a powerhead and accessories.
When people started buying just the powerhead, with an attached impeller of course,... well... there's a shorter term than, say,
submersible pump motor with impeller.
