Visible means "able to be seen." You can see things that emit or reflect light. Abstract concepts don't emit or reflect light.
Real objects can only approximate a circle. The pedantic definition of a circle in this context is being used to make the point that you can't make a perfect circle with pixels.
Real objects can only approximate a circle. The pedantic definition of a circle in this context is being used to make the point that you can't make a perfect circle with pixels.