A tuning fork is a small steel instrument which is used to tune instruments by striking it against something toproduce a note of fixed musical pitch.
tuning fork in British English
noun
a two-pronged metal fork that when struck produces a pure note of constant specified pitch. It is used to tune musical instruments and in acoustics
tuning fork in American English
a small steel instrument with two prongs, which when struck sounds a certain fixed tone in perfect pitch: it is used as a guide in tuning instruments, in testing hearing, etc.