Byte order mark
The byte order mark (BOM) is a particular usage of the special Unicode character code, U+FEFF ZERO WIDTH NO-BREAK SPACE, whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text: the byte order, or endianness, of the text stream in the cases of 16-bit and 32-bit encodings; the fact that the text stream's encoding is Unicode, to a high level of confidence; which Unicode character encoding is used. BOM use is optional.