Least significant bit

The least significant bit is the binary digit in the position of weight $2^0$. For a nonnegative integer $n$, its value is $n\mathrm{mod}2$, so it records parity. In $13=(1101{)}_2$ it is one; in $12=(1100{)}_2$ it is zero.

The phrase must be distinguished from the least significant set bit, the lowest position containing a one. For $12$, that set bit has weight four, even though the least significant bit is zero. The distinction matters in bit masks, divisibility by powers of two, and binary arithmetic.