Pitch, in sound or music, is the frequency of the sound,which moves in waves. Scientifically, this is measured as wavelength (The distance between peaks in the sound wave) or frequency (how quickly the sound waves hit your ear). Frequency is the inverse of pitch, as sound moves at a steady speed of about 722 miles per hour. A short wavelength, high frequency, is a high pitch. A long wavelength and low frequency is a low pitch. The terms "low" and "high" are arbitrary. Some other cultures call low sounds "wide," and high sounds "narrow."
Most sounds are complex waves. A pitch fork is a very clean wave, giving mostly just one pitch, and that is why it is used to tune other instruments. Each instrument has a different combination of the main pitch and overtones. That is why the same note played on different instruments (or sung by different voices) sounds different.
A pitch in baseball or softball is a ball thrown from the center of the diamond by the pitcher towards the batter. :)