At the end of a filename there is usually a 3 letter combination ( sometimes 1 or 2 or 4 letters ) following a dot. This tells the operating system what type of file it is. It therefore tells the OS how to handle the file. Examples are .HTML, .TXT, .EXE.
A sequence of characters that begins with a period (.) and is added to a filename to indicate the function or contents of the file. See also See also language extension..
A period (.) and one to three characters that can be appended to a filename. See also filename.