File Extension MASTER File Format ASP.NET Master Page MASTER stands for ASP.NET Master Page. This was last updated in March 2012 By Margaret Rouse Browse File Extensions Alphabetically: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # Ask a Question. Find an Answer. Ask your peers a question about this file extension. Ask a Question Need to make wait image while loading page? Learning ASP.NET MVC How to avoid full postback in a Master Page?