(1) See
flat-file database.
(2) A file that has no structured interrelationship between its data records. A text document without formatting structure is considered a flat file. For example, a word processing document that has been saved as "text only" is saved without markup tags or formatting elements such as bold, italic or other font specifications and information that specifies what style to make a certain line that is used as a heading. The flat file only contains the lines of text without regard to the visual presentation of the data on the page.
Flat file should not be confused with flat file system.