What is a harmonic oscillator ?

A harmonic oscillator is a one which generates frequencies which are integral multiples of fundamental frequency

it is used when there is no direct way to have such high frequency. thus a fraction of it is used. an oscillator is designed in low frequncies and then the output signal passed through a nonlinear device such a diode. output signal has harmonics of the main frequency. needed harmonic is filtered out and then amplfied. this process maybe repeated to reach to the desired frequency

just check this out, everything's got explained pretty clear!
h**p://en.wikipedia.org/wiki/Harmonic_oscillator

its a sinusoidal oscillator that has an ac outputbut no ac input

May i know LC oscillator can consider as harmonic oscillator or not? or how to implement it using electronic component?