What is resonance frequency?

A natural frequency of vibrations determined by the physical parameters of the vibrating object.

Resonance frequency; Online Definition:
In physics resonance describes the phenomenon of amplification that occurred when the frequency of a periodically applied force is harmonic proportion to a natural frequency of a system
[ phenomenon in which a vibrating system or external force drives another system to oscillate with greater amplitudes at specific frequencies ]

My definition:
Natural frequencies of vibrations determined by meters from another vibrating object :)