An octet is a group consisting of eight (8) elements, but it has several more specific meanings (usually given by a specific field or application):
Music
In music, an octet is a musical ensemble consisting of eight musicians.
The two best known octets in classical music are probably those by Felix Mendelssohn (which is for a double string quartet) and Franz Schubert (which is for clarinet, bassoon, French horn, violin, viola, cello and double bass). Igor Stravinsky also wrote an octet for wind instruments (an unusual grouping of flute, clarinet, two bassoons, two trumpets and two trombones) and Paul Hindemith wrote a lesser known piece for clarinet, bassoon, French horn, violin, two violas, cello and double bass.
See also: Octet (Reich) by Steve Reich
Computers and networking
In computer technology and networking, an octet is a group of 8 bits. It can be expressed as a decimal integer in the range 0–255, or as a pair of hexadecimal digits such as 5E (=94). In computer technology, an octet is abbreviated "o", as in "Mo" for megaoctet.
On most computers the smallest unit of memory addressing —or byte— is 8 bits, so the terms "byte" and "octet" are often used interchangeably. However, the size of a byte is determined by the architecture of a particular computer system: some old computers had 9, 10, or 12-bit bytes, while others had bytes as small as 5 or 6 bits. An octet is always exactly 8 bits. As a result, computer networking standards almost exclusively use "octet" to refer to the 8-bit quantity.
In some cases a group of 3 bits is also called an octet, as it has 8 possible values. Though one should only use octet with the meaning of 8 bits, you will often find this term e.g. for the three permission bits (read,write,execute) on Unix-like filesystems.
Chemistry
In chemistry, an atom in a chemical bond often obeys the octet rule based on the stability of 8 electrons in the valence shell.