Source : Free On-Line Dictionary of Computing

ones complement
     
        A system used in some computers to represent negative
        numbers. To negate a number, each bit of the number is
        inverted (zeros are replaced with ones and vice versa).  This
        has the consequence that there are two reperesentations for
        zero, either all zeros or all ones.
     
        	...
        	000...00011 = +3
        	000...00010 = +2
        	000...00001 = +1
        	000...00000 = +0
        	111...11111 = -0
        	111...11110 = -1
        	111...11101 = -2
        	111...11100 = -3
        	...
     
        Naive logic for ones complement addition might easily conclude
        that -0 + 1 = +0.
     
        The {twos complement} avoids this by using all ones to
        represent -1.