Per the Unicode standard, "grapheme" means "what a user thinks of as a character", i.e. nothing to do with computers, whereas "grapheme clusters" are sequences

Per the Unicode standard, "grapheme" means "what a user thinks of as a character", i.e. nothing to do with computers, whereas "grapheme clusters" are sequences of codepoints that "approximate" a grapheme (more approximately if it's EGC, less approximately if it's TGC).[1] Being able to access characters or sub-strings in constant time is useful for some (all?) character and sub-string processing. [1] http://www.unicode.org/glossary/#grapheme http://www.unicode.org/glossary/#grapheme and http://www.unicode.org/glossary/#grapheme_cluster http://www.unicode.org/glossary/#grapheme_cluster