The definition of FOUC
I prefer to resolve this abbreviation to "Flash Of Unready Content". However the "official" resolution is "Flash Of Unstyled Content". There are many variations also floating around like "FOIT" and "FOUT" which focus on specific aspects of this initial flash. Also browser vendors are discussing this matter: "The FOUC Problem".
A flash of unstyled content (FOUC) is an instance where a web page appears briefly with the browser's default styles prior to loading an external CSS stylesheet, due to the web browser engine rendering the page before all information is retrieved. The page corrects itself as soon as the style rules are loaded and applied; however, the shift is quite visible and distracting.