8 bits.

What an Intel x86 assumption! On some machines, a Byte was 6, 7, 9 or some other number of bits!

Technically, a byte is a grouping of bits that a machine deals with. Traditionally this was the number of bits required to represent a single character in the machine's native character set. Other units for measuring groupings of bits include nybbles (rare) and words (common).

On a typical modern PC, a byte is 8 bits, and a word is 4 bytes.