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 :)
Density of iron is higher than the density of oxygen because iron is solid and the particles in it are very compact but oxygen is not dense as it is gas and particles in it are loosely packed.