Positional significance
In an unsigned binary integer, the rightmost bit is the least significant bit (LSB) and represents 2⁰. Each position to the left has twice the value. The leftmost set bit is therefore the most significant bit (MSB).
The LSB also reveals parity: an integer ending in 0 is even, while one ending in 1 is odd.
Signed values and byte order
In a fixed-width two’s-complement signed integer, the highest-position bit participates in representing the sign. Its meaning depends on the chosen width, so the same visible bit pattern can represent different values under different interpretations.
MSB-first and LSB-first can also describe transmission order. Endianness is related but usually describes the order of bytes in a multi-byte value. State whether you mean bit order or byte order to avoid ambiguity.