Pitch is a perceptual attribute of sound which can be described as a sensation of the relative "height" or "altitude" of a sound. In some cultures other metaphors are used such as "large" and "small" or "long" and "short". 

The physical correlate of pitch is the frequency of the fundamental

The frequency of 440 Hz has the pitch of A above middle C.

Human perception of pitch involves a keen sensitivity to the ratio of the fundamental frequency of any two pitches. 

The most obvious example of this is our ability to perceive octave relationships between pitches. Subjectively we perceive two pitches in an octave relationship as being the same pitch only displaced from each other. Two pitches in an octave relationship to each other have a frequency ratio of 2:1, that is the frequency of the higher pitch is twice that of the lower. 

This rational perception of frequency relationships makes our ears particularly sensitive to the structure of the harmonic series. 

The harmonic series is the series of frequencies produced by a vibrating string described as integer multiples of the fundamental frequency sounded by the whole string. As the string is divided into successive integer portions (2, 3, 4, 5, etc.) a corresponding rising series of pitches of progressively smaller intervals is produced as follows: 

2:1, describing an octave 
3:2, a perfect fifth 
4:3, a perfect fourth 
5:4, a major third 
6:5, a minor third.etc. 
Our sense of pitch depends to some degree on the intensity of the tone being heard. For frequencies above 1000 Hz the perceived pitch will drop as loudness increases, while for frequencies below 1000 Hz the pitch rises as loudness increases. 
The perception of pitch is also related in subtle ways to the spectrum or timbre of a sound. The perceived pitch of a complex tone may not always correspond to frequencies actually present in the sound. An example of this is the missing fundamental phenomena whereby only the upper harmonics of some fundamental pitch may be present but the fundamental will be perceived nonetheless. 

Modern telephony takes advantage of this by band limiting voice signals to a 4000-5000 Hz range before transmitting them over telephone lines. Despite this severe loss of low frequency information where fundamental frequencies and some formant information have been lost, we can still recognise not only the pitch of a transmitted voice but identify the characteristic timbre of individual speakers.