COP — coefficient of performance — is the ratio of heat a system delivers to the electricity it consumes, both in the same units, so it is a pure number. A baseboard heater or electric furnace strip converts essentially all its electricity to heat, giving a COP of about 1. A heat pump moves existing heat rather than creating it, so it routinely delivers two to four units of heat per unit of electricity — a COP of 2 to 4 for air-source systems, which is the whole efficiency case for switching.
The catch is that COP is not constant. As the outdoor temperature falls, the heat pump has to lift heat across a bigger gap between cold outside air and warm indoor air, so the compressor works harder for each unit delivered — meaning COP is lowest exactly when heating demand is highest. This is why a system's balance point and cold-weather capacity matter as much as its season-long HSPF2 rating.
A geothermal (ground-source) heat pump sidesteps the cold-air penalty by drawing on the earth's stable 40–70°F temperature instead of winter air; the DOE puts modern ground-source COPs in the 3.9–6.0 range. COP is closely related to HSPF2 (HSPF2 is essentially a seasonal, weighted average of COP across a heating season) — COP is the instantaneous snapshot, HSPF2 the season-long summary.