Where is the diameter on a circle?

Answer 1

Any straight line that starts on the circle, ends on the circle, and goes through the center,

is a diameter.

There's no limit to the number of different diameters you can draw, all in the same circle,

but they're all the same length.

It's the longest possible straight line that you can draw inside the circle.

It's the distance 'across' anything shaped like a circle.