a key on a computer keyboard or terminal that, when pressed, alone or in combination with other keys, causes a specific computational or mechanical operation to be carried out.
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British Dictionary definitions for function key
computing a key on the keyboard of a microcomputer, etc. that gives special commands to the computer
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012