Dictionary.net

Whether a text matching operation distinguishesupper-case (capital) letters from lower case (is "casesensitive") or not ("case insensitive").Case in file names should be preserved (for readability) butignored when matching (so the user doesn't have to get itright). MS-DOS does not preserve case in file names, Unixpreserves case and matches are case sensitive.Any decent text editor will allow the user to specifywhether or not text searches should be case sensitive.Case sensitivity is also relevant in programming (mostprogramming languages distiguish between case in the names ofidentifiers), and addressing (Internet domain names arecase insensitive but RFC 822 local mailbox names are casesensitive).Case insensitive operations are sometimes said to "fold case",from the idea of folding the character code table so thatupper and lower case letters coincide. The alternative "smashcase" is more likely to be used by someone who considers thisbehaviour a misfeature or in cases where one case isactually permanently converted to the other."MS-DOS will automatically smash case in the names of allthe files you create".