A byte is a term first coined by Werner Buchholz in 1956 and later became more of a standard thanks to Bob Bemer and others. A byte is data equal to either seven or eight bits depending if it needs error correction (parity). You can think of a byte as one letter, for example, the letter ‘h’ is one byte or eight bits and the word ‘hope’ as four bytes or 32 bits (4*8).
When looking at the size of a file, a byte is the smallest measurement size listed in operating systems. However, because most files are bigger than 1,024 bytes (one kilobyte), most users will see small files listed in kilobytes.
Bytes are used not only in networking, but also for computer disks, memory, and central processing units (CPUs). In all modern network protocols, a byte contains eight bits. A few (generally obsolete) computers may use bytes of different sizes for other purposes.
The sequence of bytes in other parts of the computer may not follow the network byte order. Part of the job of the networking subsystem of a computer is to convert between the host byte order and network byte order when needed.
You can find full definition of byte at: https://en.wikipedia.org/wiki/Byte