In phonetics and phonology, gemination (), or consonant lengthening (from Latin geminatio 'doubling', itself from gemini 'twins'), 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, others describe two distinct phenomena.

Consonant length is a distinctive feature in certain languages, such as Arabic, Berber, Danish, Estonian, Hindi, Hungarian, Italian, Japanese, Kannada, Punjabi, Polish and Turkish. Other languages, such as English, do not have word-internal phonemic consonant geminates.

Consonant gemination and vowel length are independent in languages like Arabic, Japanese, Finnish and Estonian; 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. A clear example are the Norwegian words tak [tɑːk] ('ceiling or roof' of a building), and takk [tɑkː] ('thanks').