Noun
(computing) That which is not a character (unit of text).
(Unicode) One of 66 code points (the 32 code points U+FDD0–U+FDEF, plus the last two code points—U+...FFFE and U+...FFFF—of each of the 17 planes) which are guaranteed never to be assigned and are intended for process-internal use.
That which is not a character, as in literature.
Source: en.wiktionary.org