Definition: An octet, as used in the context of email technology, is a grouping of eight bits.
A bit, in turn is the most basic piece of information kept inside a computer that has two possible values — either 1 or 0, on or off, true or false. Stack eight of these bits together and you get an octet.
A group of eight bits can also be called a byte. A byte, however, can also mean any group of four to ten bits. If you say "octet", it is clear that you mean eight bits.
Email servers regularly report the size of emails not in bytes, kilo-, mega- or gigabytes but as octets.
Examples:
+OK 18 messages (67042 octets)
A typical reply from a mail server you might see in a log of POP retrieval, for example.
