11 is the natural number following 10 and preceding 12. In English, it is the smallest positive integer requiring three syllables and the largest prime number with a single-morpheme name.

Eleven derives from the Old English ęndleofon which is first attested in Bede's late 9th-century Ecclesiastical History of the English People. It has cognates in every Germanic language (for example, German elf), whose Proto-Germanic ancestor has been reconstructed as *ainlif, from the prefix *aino- (adjectival "one") and suffix *-lif- of uncertain meaning. It is sometimes compared with the Lithuanian vënólika, although -lika is used as the suffix for all numbers from 11 to 19 (analogous to "-teen").

The Old English form has closer cognates in Old Frisian, Saxon, and Norse, whose ancestor has been reconstructed as *ainlifun. This has formerly been considered derived from Proto-Germanic *tehun ("ten"); it is now sometimes connected with *leiq or *leip ("left; remaining"), with the implicit meaning that "one is left" after having already counted to ten.

While, as mentioned above, 11 has its own name in Germanic languages such as English and German, it is the first compound number in many other languages, e.g. Italian ùndici (but in Spanish and Portuguese, 16, and in French, 17 is the first compound number), Japanese 十一 jūichi.

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