Most significant bit

The most significant bit is the binary digit with greatest positional weight in a specified binary representation. For a positive integer written without leading zeros, it is the leading one: in $13=(1101{)}_2$, it has weight $2^3$.

For a fixed-width word, the term instead denotes the leftmost position even when that bit is zero. In two's-complement signed representation, that position is the sign bit. Stating the width and whether the value is signed prevents confusing the highest position of a word with the highest nonzero bit of an integer.