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Some nasty black stuff that takes hundreds of years to flow through a funnel, but can be shattered in a moment's notice if used as a baseball.
No, you were here for the music-related definition.
Pitch is how high or low a sound is. Sound travels in waves, and the waves have a characteristic frequency, that is, how many times a second the same wave pattern passes through a single point in space. That's measured in hertz (Hz), or cycles per second. The higher the frequency, the higher the sound.
In modern tuning systems, it's standard to designate the sound of a simple sine wave at 440 Hz to be the A above middle C, for music-related purposes. Going down an octave approximately divides the frequency by two; going up an octave approximately multiples it by two.
If you have a wave pattern that travels through air (or another medium), and has a given number of wave patterns per second, then you get your pitch. But why do different instruments playing the same pitch sound so different?
Because the wave patterns themselves are different. Only the frequency determines the "fundamental" pitch. Instruments, when they sound a note of a certain pitch, actually sound many pitches that, added together at different volumes, give their distinctive waveform.
The simplest, purest pitched sound is the sine wave
 . But note that, if you have a pattern that's half the length of your original wave, or a third, or a fourth, and so on, it also fits into the same wave period. These are called harmonics, and the original sine wave is called the fundamental frequency. Harmonics have half, a third, a fourth, etc. the wavelength of a the fundamental frequency, and their frequencies are correspondingly twice, thrice, etc. as high.
These different waves, all with the same fundamental period length, can be added together using a concept known as the Fourier series—basically overlain one on top of another, if you're drawing them—to create a distinctive waveform.
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