A single-byte code page is one in which each character (or other symbol) has a numeric value expressible in a single byte.
A double-byte code page is one in which each character (or other symbol) has a numeric value expressible in a maximum of two bytes.
A triple-byte code page is one in which each character (or symbol) has a numeric value expressible in a maximum of three bytes.
An n-byte code page is one in which each character (or symbol) has a numeric value expressible in a maximum of n bytes. For example, UTF-8 is a 4-byte code page
A multi-byte code page is a code page that is not single byte.