a prime number!
A prime number is basically a number that is only by itself and one divisible. Further condition is that the number is greater than 1.
Since time immemorial, humans and computers calculate larger and larger primes. The current record is a number with 17425170 decimals (as of 2013).
Primes are the base for many other calculations in mathematics and are deeply manifested in the history of mankind. Primes have been discovered by the ancient Greeks. They are mainly used for cryptography.