twin

Twin is a term with multiple senses in English. Most commonly, it denotes one of two offspring produced in a single pregnancy. It can also function as an adjective describing two things that are closely related, paired, or similar.

Etymology: the word originates from Old English twinn, from Proto-Germanic roots, with meaning 'twofold'.

In biology, twins are classified by zygosity. Monozygotic (identical) twins result from a single fertilized egg that splits; dizygotic (fraternal) twins develop from two separate eggs fertilized by separate sperm. The incidence varies by population and factors such as maternal age and fertility treatments.

In mathematics and physics, the term appears in several contexts. Twin primes are pairs of primes with a difference of two, e.g., 11 and 13. The twin prime conjecture posits that there are infinitely many such pairs. The twin paradox is a thought experiment in special relativity about time dilation for a traveler moving at high speed.

As an adjective, twin is used in phrases such as twin beds, twin engines, or twin towns and cities. In geography and culture, "Twin Cities" typically refers to paired urban centers like Minneapolis–Saint Paul in the United States, and in mythology the Dioscuri are known as the Twins Castor and Pollux. In astrology, Gemini is the sign represented by twins.