Frequency is the speed at which a wave of energy completes a cycle of oscillation or vibration in one second, and it is measured in cycles per second or Hertz (Hz). The greater the frequency is within the audible range, the higher the pitch. Doubling the frequency increases the pitch by one octave, but the tone chroma remains the same, and, likewise, halving the frequency reduces the pitch by one octave but the tone chroma remains unchanged.
Frequency is a physical aspect of sound and can thus be measured objectively, whereas pitch is a perception of sound by humans and consequently can only be measured subjectively. The perceived pitch is generally proportional to the frequency, although there are exceptions, such as a slight change as the loudness of the sound increases.