A semiprime is the product of two (possibly equal) prime numbers.
Other names for a "semiprime" are biprime, 2-almost prime, and pq-number.
Interestingly, though the Goldbach conjecture in its full form remains intractable to current techniques, a related related result for semiprimes has been known since the 1970s. The work of Chen Jingrun showed that every even number is either the sum of two primes, or the sum of a prime and a semiprime. It has also been shown that there exist infinitely many primes p such that p+2 is either a prime or a semiprime. Primes that have this property are known as Chen primes. This fact is an analogue of the twin primes conjecture for semiprimes, just as the Goldbach conjecture is also known to be true for semiprimes.
- Semiprimes at the OEIS