In phonetics and phonology, gemination (/ˌdʒɛmɪˈneɪʃən/ ^ⓘ; from Latin geminatio 'doubling', itself from gemini 'twins'), or consonant lengthening, is an articulation of a consonant for a longer period of time than that of a singleton consonant. It is distinct from stress. Gemination is represented in many writing systems by a doubled letter and is often perceived as a doubling of the consonant. Some phonological theories use 'doubling' as a synonym for gemination, while others describe two distinct phenomena.

Consonant length is a distinctive feature in certain languages, such as Japanese. Other languages, such as Modern Greek, do not have word-internal phonemic consonant geminates.

Consonant gemination and vowel length are independent in languages like Arabic, Japanese, Hungarian, Malayalam, and Finnish; however, in languages like Italian, Norwegian, and Swedish, vowel length and consonant length are interdependent. For example, in Norwegian and Swedish, a geminated consonant is always preceded by a short vowel, while an ungeminated consonant is preceded by a long vowel. In Italian, a geminate is always preceded by a short vowel, but a long vowel precedes a short consonant only if the vowel is stressed.

Lengthened fricatives, nasals, laterals, approximants and trills are simply prolonged. In lengthened stops, the obstruction of the airway is prolonged, which delays release, and the closure is lengthened. That is, /tʰː/ is pronounced [tːʰ], not *[tʰː]. In affricates, it is also the closure that is lengthened, not the fricative release. That is, /t͜sː/ is pronounced [tːs], not *[tsː].

In terms of consonant duration, Berber and Finnish are reported to have a 3-to-1 ratio, compared with around 2-to-1 (or lower) in Japanese, Italian, and Turkish.

Gemination of consonants is distinctive in some languages and then is subject to various phonological constraints that depend on the language.

In some languages, like Italian, Swedish, Faroese, Icelandic, and Luganda, consonant length and vowel length depend on each other. A short vowel within a stressed syllable almost always precedes a long consonant or a consonant cluster, and a long vowel must be followed by a short consonant. In Classical Arabic, a long vowel was lengthened even more before permanently-geminate consonants.

In other languages, such as Finnish, consonant length and vowel length are independent of each other. In Finnish, both are phonemic; taka /taka/ 'back', takka /takːa/ 'fireplace' and taakka /taːkːa/ 'burden' are different, unrelated words. Finnish consonant length is also affected by consonant gradation. Another important phenomenon is sandhi, which produces long consonants at word boundaries when there is an archiphonemic glottal stop |otaʔ se| > otas se 'take it (imperative)!'.