[justinpombrio]

I would describe an illusion as something where my (say) visual cortex is lying to me. This results in an image looking very odd: I _know_ that one thing is happening, but it looks like something else is happening.

Black text going in front of a black background doesn't have that effect. My knowledge of what I'm seeing agrees with what I see. My knowledge is "there is black text in front of that black text", and my visual cortex says "sure, I can't see it but that's perfectly plausible". This pigeon video does have that effect. My knowledge says that the pigeon image is moving smoothly, but I can clearly see its head bobbing; there's a disagreement.

Do other people define "illusion" differently? Or perceive something differently?

[lupire]

It's th e same illusion as someone walking out a door and somehow teleporting to another door after they walked around outside.