"Emirp" is the word "prime" spelled backwards, and it refers to a prime number that becomes a new prime number when you reverse its digits. Emirps do not include palindromic primes (like 151 or 787), nor 1-digit primes like 7. The first few emirps are 13, 17, 31, 37, 71, 73, 79, 97, 107, 113, 149, and 157 – reverse them and you have got a new prime number on your hands.