What is the definition or description of: peripheral vision?

Not central.: Peripheral vision refers to any vision one has that is not central. Peripheral vision is usually measured in degrees from your central vision. Different conditions can cause loss of peripheral like glaucoma, strokes, a brain mass etc.

Side Vision: When you look at someone's face, you can still see their hand, even though you are not looking directly at it. When you are walking through a doorway, you are not looking at the sides, or looking directly at the threshold, but you see it through your peripheral vision. Anything outside of your direct focus spot, is considered peripheral. Which can be measured by "visual field examination".