A mixture of single-byte characters from a single-byte character set and double-byte characters from a double-byte character set. See single-byte character set, double-byte character set.
A multibyte character is a character that takes up several buffer positions. Emacs uses multibyte characters to represent non-ASCII text, since the number of non-ASCII characters is much more than 256. See section Introduction to International Character Sets.
(n.) A character whose codepoint is stored in 1 or more bytes. It differs from wide-character encoding in that the number of bytes representing a character can vary.