I understand that it IS pi, but I don’t understand what “ratio of the circumference of any circle to the diameter” means.
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Imagine the circle is the top of a barrel. You can take a measuring tape and wrap it around the top to measure the outside – the circumference. You could also use the measuring tape to find the distance across the circular top – the diameter.
If you divide the number you measured for the circumference by the number you measured for the diameter (that’s a ratio), you get Pi (3.141592…). It doesn’t matter how big the circle is, the circumference is always Pi times as big as the diameter.
That’s how we define Pi. When people recognized that the circumference divided by the diameter of the circle never changed (it is “a constant”) they figured that was a useful number to know and used the greek letter as a nickname for it.
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