Define tempering in brief

Short answer

Tempering is reheating quenched steel to 150-650°C, holding and air/oil cooling. It reduces brittleness and internal stresses of martensite, increasing toughness and ductility while some hardness is lost.

Define tempering.

Tempering is the reheating of hardened (quenched) steel to a temperature below the lower critical point (150-650 C), holding it there, and then cooling in air or oil. It reduces the brittleness and internal stresses of martensite and increases toughness and ductility, with some loss of hardness.

 Temp
  ^
  |        ______ temper temp (150-650 C)
  |       /      \
  |      /        \ cool in air
  |     /          \
  +--------------------------> Time

Lower tempering temperature (150-300 C) retains hardness (cutting tools); higher (400-650 C) gives more toughness (springs, shafts).

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