the.com/space heater

means A small portable electric or fuel-burning unit that heats the immediate area around it, rather than an entire house.

from Straightforward compound coinage: 'space' in the sense of a localized area or room, plus 'heater,' an agent noun from 'heat' meaning the thing that does the heating. It rose alongside 20th-century electrification, as portable resistive-coil and later ceramic heaters became cheap household goods — the name simply describes the job: heating a 'space' rather than a whole building, the way a furnace or central heating system would.