In phonetics, gemination is when a spoken consonant is "doubled", so that it is pronounced for an audibly longer period of time than a "single" consonant. The term comes from the word Gemini, the astrological sign for the Twins. In written language, this is often indicated by writing a consonant twice ("ss", "kk", "pp", etc.), but can also be indicated with a special symbol, such as the shadda in Arabic. In some languages, such as Italian, Japanese, Arabic and Finnish, this can actually affect the meaning of words, where the only difference between one word and another is whether a consonant is pronounced singly or doubly (geminated).

In English, phonetic gemination is rare but does occur. It is found where a root-word is preceded by another root or a prefix ending with the same letter or sound that the second root begins with. Examples: "homemade," "screenname," "flat-top," "misspell," "unknown," "interrelated," "innumerable", "irredeemable." It is also found when the suffix -ly follows a root ending in -l or -ll. Examples: "fully", "evilly", "dully", "foully." Naturally, it also occurs over word boundaries: "I'll learn", "some money", "with things".

In most instances (still speaking about English), the absence of this doubling does not affect the meaning, though it may confuse the listener momentarily. Notable examples where the doubling does affect the meaning are the pairs "unaimed" versus "unnamed", and "holy" versus "wholly".

Gemination can also be a purely spelling phenomenon, as in English words like "running" where there is no lengthening of the consonant in actual speech.