What is the definition of oscillator?

A circuit that produces an alternating output current of a certain frequency determined by the characteristics of the circuit components.

Generally speaking however; An oscillator moves from its highest state, to its lowest state in quick succession, A pendulum can be considered an oscillator.
The speed of the alterations is called the frequency, and the 'distance' traveled can be related to wavelength of the wave or oscillation.

Resonate frequency is the point at which resistance to the oscillations is at its absolute minimum and the amplitude of the wave is greatly increased, this is because the wave tends to store energy, or rather when encountering resistance the wave is not perfect and has 'ripples' or 'small spikes' within the wave, after the wave reaches resonance it becomes smooth and all that built up energy is forced into the cycle of the wave.