What is the Pi the ratio of?
It is the ratio of the circumference of a circle to it's diameter. (C/D=pi)
If you have a circle ... any circle ... and you measure its circumference, measure its diameter, and divide the circumference by the diameter, the answer is always the same number, which people have decided to label with the name "pi". "Pi" is the ratio of a circle's circumference to its diameter, but "pi" is not a rational number, since it's not the ratio of any two integers.
pi is the square root of ten ---------- Incorrect. Pi is the mathematical ratio between the circumference and the diameter of a circle. Coincidentally, it is also the ratio between the area of a circle and the square of its radius. The square root to 10 is ~3.16227766, while Pi is ~3.1415926535897932