least significant bit,and most significant bit
"Most significant bit" is a concept that depends on the size of the machine word. If $a + b + c = 10$ and the machine word has size $64$, then the MSB is $0$. It is $1$ only if the machine word has size $2$. (Under your definition, the MSB is always $1$, unless the number is $0$, in which case it is undefined.)