That would be a
hallucination - "A sensory perception of something that does not exist."
Optical illusions are just the most familiar type of illusion. Other types of illusion (e.g. the McGurk Effect) are often more revealing.
Our brains make continual adjustments to our perceptions to make them coherent with our expectations; for example, when you touch your finger to your nose, it feels like the finger and the nose sense the touch at the same time, but it actually takes significantly longer for the signal to get from your finger to your brain than from your nose to your brain - so the brain synchronizes the signals by delaying the signal from the nose. Similarly, when you watch someone making a noise by hitting something (a nail, a baseball, etc), or bouncing a ball, the brain synchronises the sound with your vision up to about 50 metres, beyond which you start to notice a distinct gap between the sight of the impact and the sound.
For sensory events that happen rarely or where the synchronisation isn't particularly important, the differences are noticeable - when you stub your toe, you feel the impact right away because touch signals travel at 250 feet per second. But you don't feel the pain for two or three seconds, because pain signals from the foot travel at only 2 feet per second - sometimes it's long enough for you to think, "Uh-oh, this is gonna hurt!".
We have the impression that our visual field is all high-resolution and full colour, but only a tiny region (about the size of a thumbnail at arm's length) actually is; the rest is filled in from an internal model of what we expect to see, which is corrected and updated about three times a second by the scrappy image our eyes actually provide.
The image the eye makes before processing:
Our eyes are constantly jumping around taking in different parts of the scene ('
saccades'), but these sudden movements are edited out of the final perception, so you don't notice them visually - you're more likely to feel them occasionally as your eyeballs move in their sockets.
Like it or not, our perception is a collection of heuristics, shortcuts, and guesses, dressed up to feel like accurate high-resolution sensing of the world.
Trust it at your own risk.