Phonology (formerly also phonemics or phonematics) is the branch of linguistics that studies how languages systematically organize their phonemes or, for sign languages, their constituent parts of signs. The term can also refer specifically to the sound or sign system of a particular language variety. At one time, the study of phonology related only to the study of the systems of phonemes in spoken languages, but now it may relate to any linguistic analysis either:

Sign languages have a phonological system equivalent to the system of sounds in spoken languages. The building blocks of signs are specifications for movement, location, and handshape. At first, a separate terminology was used for the study of sign phonology ("chereme" instead of "phoneme", etc.), but the concepts are now considered to apply universally to all human languages.

The word "phonology" (as in "phonology of English") can refer either to the field of study or to the phonological system of a given language. This is one of the fundamental systems that a language is considered to comprise, like its syntax, its morphology and its lexicon. The word phonology comes from Ancient Greek φωνή, phōnḗ, 'voice, sound', and the suffix -logy (which is from Greek λόγος, lógos, 'word, speech, subject of discussion').

Phonology is typically distinguished from phonetics, which concerns the physical production, acoustic transmission and perception of the sounds or signs of language. Phonology describes the way they function within a given language or across languages to encode meaning. For many linguists, phonetics belongs to descriptive linguistics and phonology to theoretical linguistics, but establishing the phonological system of a language is necessarily an application of theoretical principles to analysis of phonetic evidence in some theories. The distinction was not always made, particularly before the development of the modern concept of the phoneme in the mid-20th century. Some subfields of modern phonology have a crossover with phonetics in descriptive disciplines such as psycholinguistics and speech perception, which result in specific areas like articulatory phonology or laboratory phonology.

Definitions of the field of phonology vary. Nikolai Trubetzkoy in Grundzüge der Phonologie (1939) defines phonology as "the study of sound pertaining to the system of language," as opposed to phonetics, which is "the study of sound pertaining to the act of speech" (the distinction between language and speech being basically Ferdinand de Saussure's distinction between langue and parole). More recently, Lass (1998) writes that phonology refers broadly to the subdiscipline of linguistics concerned with the sounds of language, and in more narrow terms, "phonology proper is concerned with the function, behavior and organization of sounds as linguistic items." According to Clark et al. (2007), it means the systematic use of sound to encode meaning in any spoken human language, or the field of linguistics studying that use.

Evidence for a systematic investigation of the sounds of a language appears in the 4th c. BCE Ashtadhyayi, a Sanskrit grammar by Pāṇini. Particularly, within the Shiva Sutras, auxiliary work to the Ashtadhyayi, an inventory of what would be construed as a list of the phonemes of Sanskrit is provided, with a notational scheme for them which is deployed throughout the main text, which concern itself with issues of morphology, syntax and semantics.

Ibn Jinni of Mosul, a pioneer in phonology, wrote prolifically in the 10th century on Arabic morphology and phonology in works such as Kitāb Al-Munṣif, Kitāb Al-Muḥtasab, and Kitāb Al-Khaṣāʾiṣ  [ar].

The study of phonology as it exists today is defined by the formative studies of the 19th-century Polish scholar Jan Baudouin de Courtenay, who (together with his students Mikołaj Kruszewski and Lev Shcherba in the Kazan School) shaped the modern usage of the term phoneme in a series of lectures in 1876–1877. The word phoneme had been coined a few years earlier, in 1873, by the French linguist A. Dufriche-Desgenettes. In a paper read at 24 May meeting of the Société de Linguistique de Paris, Dufriche-Desgenettes proposed for phoneme to serve as a one-word equivalent for the German Sprachlaut. Baudouin de Courtenay's subsequent work, though often unacknowledged, is considered to be the starting point of modern phonology. He also worked on the theory of phonetic alternations (what is now called allophony and morphophonology) and may have had an influence on the work of Saussure, according to E. F. K. Koerner.