[lutorm]

By that definition, nothing is visible. You never see anything except photons.

[Benjo]

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.

[lutorm]

No. If you made an abstract object that emitted light, like the "linear light source" I hypothesized, it would be a perfect circle, and visible. It doesn't exist, of course, but if it did, it would be visible... ;-)

[thebooktocome]

That's true; and sometimes, that definition is useful, as it is here.