In linguistics, a grapheme is the smallest functional unit of a writing system. The word grapheme is derived from Ancient Greek's gráphō ('write'), and the suffix -eme (by analogy with phoneme and other emic units). The study of graphemes is called graphemics. The concept of a grapheme is abstract; it is similar to the notion of a character in computing. (A specific geometric shape that represents any particular grapheme in a given typeface is called a glyph.) In orthographic and linguistic notation, a particular glyph (character) is represented as a grapheme (is used in its graphemic sense) by enclosing it within angle brackets: e.g. ⟨a⟩.

There are two main opposing grapheme concepts.

In the so-called referential conception, graphemes are interpreted as the smallest units of writing that correspond with sounds (more accurately phonemes). In this concept, the sh in the written English word shake would be a grapheme because it represents the phoneme /ʃ/. This referential concept is linked to the dependency hypothesis that claims that writing merely depicts speech.

By contrast, the analogical concept defines graphemes analogously to phonemes, i.e. via written minimal pairs such as shake vs. snake. In this example, h and n are graphemes because they distinguish two words. This analogical concept is associated with the autonomy hypothesis which holds that writing is a system in its own right and should be studied independently from speech. Both concepts have weaknesses.

Some models adhere to both concepts simultaneously by including two individual units, which are given names such as phonological-fit grapheme for the grapheme according to the referential concept (sh in shake), and graphemic grapheme for the grapheme according to the analogical conception (h in shake).

In newer concepts, in which the grapheme is interpreted semiotically as a dyadic linguistic sign, it is defined as a minimal unit of writing that is both lexically distinctive and correspondent to a linguistic unit (phoneme, syllable, or morpheme).

Graphemes are often notated within angle brackets: e.g. ⟨a⟩. This is analogous to the slash notation /a/ used for phonemes. Analogous to the square bracket notation [a] used for phones, glyphs are sometimes denoted with vertical lines, e.g. |ɑ|.

In the same way that the surface forms of phonemes are speech sounds or phones (and different phones representing the same phoneme are called allophones), the surface forms of graphemes are glyphs (sometimes graphs), namely concrete written representations of symbols (and different glyphs representing the same grapheme are called allographs).