transitive verb
Definition 1
Bring metal, especially steel, to proper hardness and flexibility by successive heating and cooling.
Source: Concise Oxford Dictionary
Context: To reduce brittleness and to relieve internal stresses, steel must be tempered after it has been hardened.
Source: Basic Optics and Optical Instruments, Fred A. Carson (Google Books)
Definition 2
Moderate, restrain, tone down.
Source: Concise Oxford Dictionary
Context: Lewis Namier once called 18th-century England "an aristocracy tempered by riots".
Source: Financial Times 23/02/2010 (Tony Barber)