-A file extension tells the computer and user what kind of information is stored within a file. The file extension is normally the last three characters in a filename preceded by a '.' dot.
http://www.le.ac.uk/webcentre/help/glossary.html
-the portion of a filename, following the final dot, which indicates the kind of data stored in the file
http://en.wikibooks.org/wiki/SA_NCS_Computer_Application_Technology:Glossary
-Three letters at the end of a filename that indicate the type of information stored in the file. For example, the file myfile.doc has a file extension of doc meaning it is a Microsoft Word Document.
http://web.mit.edu/abiword_v2.4.5/Tutorials/klw/glossary.html
-The letters following the full stop in a file name. The file extension usually identifies the type of data found in the file.
http://data.bolton.ac.uk/bissto/glossary/index.htm
-A group of characters appended to the end of a disk file name. File extensions usually consists of a full stop (dot) and one to three characters.
http://www.centc251.org/Ginfo/Glossary/tcglosf.htm
-Under DOS naming conventions, the file extension is the three character portion that follows the file name and identifies the type of file.
http://www.pccomputernotes.com/pcterms/glossaryf.htm
-The character string after the right-most period in a filename. File extensions are used to label files by their type, origin, and possible uses.
http://wwwlb.aub.edu.lb/~acc/Resources/glossarycommonterms.htm
-A set of characters added to a filename that serves to extend or clarify its meaning or to identify a file as a member of a category.
http://www.doc-o-matic.com/webhelp/!!GLOSSARY.html
-A file extension is nothing more than the last characters after the period in the name of a file. The extension defines the file "type", eg text file, executable file, database file or graphic file.
http://www.optimumrecovery.com/glossary/filerecovery-glossary.html