Longman Dictionary of Contemporary English
Wiktionary
n. (context computing English) A key on a personal computer or terminal keyboard used in combination to modify the function of other keys; normally labelled "Ctrl"
WordNet
n. (computer science) the key on a computer keyboard that is used (in combination with some other key) to type control characters [syn: command key]
Wikipedia
In computing, a Control key is a modifier key which, when pressed in conjunction with another key, performs a special operation (for example, ); similar to the Shift key, the Control key rarely performs any function when pressed by itself. The Control key is located on or near the bottom left side of most keyboards (in accordance with the international standard ISO/IEC 9995-2), with many featuring an additional one at the bottom right.
It is usually labeled (rarely, or is seen) on keyboards which use English abbreviations for key labeling. Abbreviations in the language of the keyboard layout also are in use. e.g. the German layout uses as required by the German standard DIN 2137:2012-06. Also, there is a standardized keyboard symbol (to be used when Latin lettering is not preferred), given in ISO/IEC 9995-7 as symbol 26, and in ISO 7000 “Graphical symbols for use on equipment” as symbol ISO-7000-2028. This symbol is encoded in Unicode as U+2388 (⎈).