Dictionary.net

A character which is input or output to cause somespecial action rather than to appear as part of the data.Most control characters are input by holding down (either of)the "Control" key(s) on the keyboard and simultaneouslypressing another key which may be a letter or (depending onthe keyboard and operating system) certain punctuationcharacters. Some control codes usually have their own specialkeys: escape, tab, delete, backspace, return and canthus be entered with a single keypress.Different operating systems and programs have differentconventions for what effect typing certain control codes willhave, such as interrupting the current process or suspendingoutput.Control codes may be output for their effect on the outputdevice, e.g. moving the cursor or print head to the start of anew line.See control-O, control-Q, control-S.