A Chen prime is a prime \(p\) such that \(p + 2\) is either prime or a semiprime (a product of two primes). Every twin prime's smaller member is automatically a Chen prime; the class also includes primes like 23, where 25 = 5² is a semiprime.
The name honors Chen Jingrun, whose 1966 theorem — every sufficiently large even number is the sum of a prime and a number with at most two prime factors — remains the closest anyone has come to proving Goldbach's conjecture. The same paper proved there are infinitely many Chen primes, a result standing in for the still-open twin prime conjecture.