Two prime numbers having a difference of 2 are known as: 

Two prime numbers that have a difference of 2 are known as "twin primes." Twin primes are pairs of prime numbers that have a gap of 2 between them. 
The first few twin prime pairs are (3, 5), (5, 7), (11, 13), (17, 19), and so on.
For example, in the pair (11, 13), the difference between the two primes is 13 - 11 = 2. 
 

Therefore, the correct answer is twin prime.