Source : Free On-Line Dictionary of Computing

bit mask
     
         A pattern of {binary} values which is combined
        with some value using {bitwise} AND with the result that bits
        in the value in positions where the mask is zero are also set
        to zero.  For example, if, in {C}, we want to test if bits 0
        or 2 of x are set, we can write
     
        	int mask = 5;	/* binary 101 */
     
        	if (x & mask) ...
     
        A bit mask might also be used to set certain bits using
        bitwise OR, or to invert them using bitwise {exclusive OR}.
     
        (1995-05-12)