Is it ensure, insure, or assure?
A character which is input or output to cause some special 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 simultaneously pressing another key which may be a letter or (depending on the keyboard and operating system) certain punctuation characters. Some control codes usually have their own special keys: escape, tab, delete, backspace, return and can thus be entered with a single keypress.
Different operating systems and programs have different conventions for what effect typing certain control codes will have, such as interrupting the current process or suspending output.
Control codes may be output for their effect on the output device, e.g. moving the cursor or print head to the start of a new line.
See control-O, control-Q, control-S.