The Free On-line Dictionary of Computing (30 December 2018):

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)