From Teflpedia

A grapheme (/græfi:m/) is the smallest unit of a writing system of any given language.[1] An individual grapheme may or may not carry meaning by itself, and may or may not correspond to a single phoneme of the spoken language. Graphemes include alphabetic letters, typographic ligatures, numerical digits, punctuation marks, and other individual symbols.

Some linguists consider digraphs and trigraphs to be distinct graphemes, but these are generally analysed as sequences of graphemes.

The study of graphemes is graphemics.

In analysis, graphemes are often marked by round brackets, e.g (a) (b) (1) (2) (&), etc