Byte order mark
termU+FEFF at the very start of a text: it says which byte order a UTF-16 or UTF-32 text uses, and serves as a signature that a text is UTF-8.
In UTF-16 it reads FE FF in big-endian order and FF FE in little-endian; in UTF-8 it is EF BB BF and says nothing about byte order, only that the text is very probably UTF-8. A browser trusts it over any charset label. Elsewhere it is often unwelcome: it is invisible in most editors, RFC 8259 forbids adding one to JSON sent over a network, and a parser that does not expect one sees an extra character (JavaScript's JSON.parse rejects a text that starts with one).
Also known as: bom, byte-order mark, u+feff