An emirp is a prime number that in a given base $b$ reads like a different prime number backwards. The plural of emirp is emirps, but semirp is occasionally used by amateurs.

For example, in base 10, the number 37 read backwards is 73, both of these are prime. The backwards number must be different. A006567 of Sloane's OEIS lists the first fifty base 10 emirps.

A palindromic prime is not an emirp, since it reads the same number both ways. Thus, in binary, Mersenne primes can not be emirps.