The term "octet" is used in IETF documentation (for IP addresses, for example) to be specific that the byte is 8 bits in length. Historically the size of a "byte" on a system was machine-dependent. The industry coalesced around the 8-bit byte, and differentiated it from "machine word" in the 70s and 80s.

Edit: I just checked wikipedia, and this is described there: https://en.wikipedia.org/wiki/Byte