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Depending on context, either the most signifigant byte or most signifigant bit. For example, in a 32-bit word, the MSBit is equal to 1<<31 == 2^31 == 2147483748, whereas the MSByte is equal to i<<24 == i*(1<<24) == i*16777216, where i is the value of the byte; for example, if i==1, MSByte==16777216.

Contrast this to the LSB, which is always equivalent to 1 (whether it's the LSBit or LSByte).

Both of these definitions are assuming 2's complement and/or unsigned integer values. The meanings change signifigantly (no pun intended) when applied to 1's complement or signed magnitude or fixed point or whatever.