[AIVC, Air infiltration and ventilation glossary, 1992]:
the proportion of heat recovered from otherwise waste heat passing through a heat recovery system. Normally expressed as a percentage

[EN 14511-1:2022, 3.22]:
  (efficiency)ratio of the total capacity of the system (heating plus cooling capacity) to the effective power input when operating in the heat recovery modeNOTE Expressed in kW/kW.