File Extension INDEX File Format Generally index file INDEX stands for Generally index file. 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 What is difference between Clustered Index and Non-Cluster Index? Difference between clustered and non-clustered index How do search engines index image files?