Source : Free On-Line Dictionary of Computing

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        1. The {out-of-band} value returned by {C}'s sequential
        character-input functions (and their equivalents in other
        environments) when end of file has been reached.  This value
        is -1 under {C} libraries postdating V6 Unix, but was
        originally 0.
     
        2. The keyboard character (usually control-D, the ASCII EOT
        (End Of Transmission) character) that is mapped by the {Unix}
        terminal driver into an end-of-file condition.
     
        [{Jargon File}]
     
        (1995-01-18)