Foveal viewport (Foveal area)

The fovea, a part of human's eye, is responsible for sharp central vision, necessary in humans for reading, watching television or movies, driving, and any activity where visual detail is of primary importance. Foveal area is a small wide space area where your eyes are aimed at and it is the only area where you can perceive the maximum level of detail. Foveal area is a tight area of about two degrees of visual field or two thumbnails held in front of your eyes. This is the place where you'd like to deliver the most important messages of your visitors.

Foveal viewport is important, because outside of this wide screenarea how your visitors see your web-pages change dramatically. Inside this area is the only part of your vision with the maximal resolution -only here no eye scanning is necessary. [Source]