A pair of prime numbers that differ by 2. For example, 29 and 31, 71 and 73, and 10 006 427 and 10 006 429, are twin primes. The conjecture that there are infinitely many such pairs remains an open problem.

http://primes.utm.edu/top20/page.php?id=1 More information on twin primes, including a list of the twenty largest pairs identified.