In sociolinguistics, digraphia refers to the use of more than one writing system for the same language. Synchronic digraphia is the coexistence of two or more writing systems for the same language, while diachronic digraphia or sequential digraphia is the replacement of one writing system by another for a particular language.

Hindustani, with an Urdu literary standard written in Urdu alphabet and a Hindi standard written in Devanagari, is one of the "textbook examples" of synchronic digraphia, cases where writing systems are used contemporaneously. An example of diachronic digraphia, where one writing system replaces another, occurs in the case of Turkish, for which the traditional Arabic writing system was replaced with a Latin-based system in 1928.

Digraphia has implications in language planning, language policy, and language ideology.

English digraphia, like French digraphie, etymologically derives from Greek di- δι- "twice" and -graphia -γραφία "writing".

Digraphia was modeled upon diglossia "the coexistence of two languages or dialects among a certain population", which derives from Greek diglossos δίγλωσσος "bilingual." Charles A. Ferguson, a founder of sociolinguistics, coined diglossia in 1959. Grivelet analyzes how the influence of diglossia on the unrelated notion of digraphia has "introduced some distortion in the process of defining digraphia," such as distinguishing "high" and "low" varieties. Peter Unseth notes one usage of "digraphia" that most closely parallels Ferguson's "diglossia," situations where a language uses different scripts for different domains; for instance, "shorthand in English, pinyin in Chinese for alphabetizing library files, etc. or several scripts which are replaced by Latin script during e-mail usage."

The Oxford English Dictionary, which does not yet include digraphia, enters two terms, digraph and digraphic. First, the linguistic term digraph is defined as, "A group of two letters expressing a simple sound of speech". This meaning applies to both two letters representing a single speech sound in orthography (e.g., English ng representing the velar nasal /ŋ/) and a single grapheme with two letters in typographical ligature (e.g., the Old English Latin alphabet letter æ). Second, the graph theory term digraph (a portmanteau from directed graph) is defined as, "A graph in which each line has a direction associated with it; a finite, non-empty set of elements together with a set of ordered pairs of these elements." The two digraph terms were first recorded in 1788 and 1955, respectively. The OED2 defines two digraphic meanings, "Pertaining to or of the nature of a digraph" and "Written in two different characters or alphabets." It gives their earliest examples in 1873 and 1880 (which was used meaning "digraphia"). Isaac Hollister Hall, an American scholar of Oriental studies, described an Eteocypriot language publication as "bilingual (or digraphic, as both inscriptions are in the same language)." Hall's article was antedated by Demetrios Pieridis's 1875 usage of digraphic instead of bilingual for an inscription written in both the Greek alphabet and Cypriot syllabary.

English digraphic and digraphia were contemporaneous with their corresponding terms in French linguistics. In 1877, Julius Oppert introduced digraphique to describe languages written in cuneiform syllabaries. In 1893, Auguste Barth used French digraphisme for Cambodian inscriptions written in Khmer script and Brāhmī script. In 1971, Robèrt Lafont coined digraphie regarding the sociolinguistics of French and Occitan.

Although the word "digraphia" is new, the practice is ancient. Darius the Great's (c. 522-486 BCE) Behistun Inscription was written in three cuneiform scripts for Old Persian, Elamite, and Babylonian.