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##### Wiktionary
twin prime

n. (context mathematics English) Either of a pair of prime numbers that differ by two; example $5$ and $7$ as well as $191$ and $193$

##### Wikipedia
Twin prime

A twin prime is a prime number that is either 2 less or 2 more than another prime number —for example, the twin prime pair ( 41, 43). In other words, a twin prime is a prime that has a prime gap of two. Sometimes the term twin prime is used for a pair of twin primes; an alternative name for this is prime twin or prime pair.

Twin primes become increasingly rare as one examines larger ranges, in keeping with the general tendency of gaps between adjacent primes to become larger as the numbers themselves get larger. However, it is a longstanding conjecture that there are infinitely many twin primes. Work of Yitang Zhang in 2013, as well as work by James Maynard, Terence Tao and others, has made substantial progress towards proving this conjecture, but at present it remains unsolved.