Definition: The natural eye movement by Western societies when scanning content rich web pages. Most people will automatically scan the top of a page, then skim down the left-hand side and make a few occasional forays into the center.
Reference: The pattern appears during eye tracking sessions that use heat maps to track a user's gaze. This word is best described with a picture so here's one!
It is important to keep the F-Pattern in mind while laying out a page because this will ensure that the most important UI elements will easily and quickly be seen by your users.