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.