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.