The Doppler Effect refers to the change in frequency and wavelength of a wave when either the source of the wave or the observer is in motion relative to each other. This causes a shift in pitch or frequency perception for sound waves.
The number of complete cycles or oscillations per second that a wave completes. It determines the pitch of a sound.
The distance between two corresponding points on a wave (crest to crest or trough to trough). It affects both pitch and timbre of a sound.
How high or low we perceive a sound to be based on its frequency.