Pitch, in physics, is equivalent to the frequency of sound waves, which are any compression waves in a medium. The word "pitch" is used specifically in situations where humans are perceiving the frequency of sound and changes in that frequency. Differences and changes in perceived pitch aren't always the same as the actual differences and changes in the frequencies of the sounds perceived.
In general, the perception of higher pitches and lower pitches is accompanied by higher and lower frequencies of sound, but this isn't actually always the case. For instance, high pitches are perceived to grow higher as they grow louder, and low pitches are perceived to grow lower as they grow louder.
The frequency of sound waves isn't referred to as pitch in situations where it is purely measured by scientific instruments. Indeed, the smallest unit of pitch is called a cent, which means the smallest difference in pitch detectable by the human ear. Different people have different abilities to perceive and identify pitch. A tiny proportion of the population actually has perfect pitch, the ability to identify a pitch without reference to any other standard pitch. This ability is far less developed in humans than the perception of color, which is the equivalent perception of the frequency of light.