pi(x) is the number of primes less than or equal to x
Let x be a positive real number. The question "how many
primes are there less than x?" has been asked so frequently that its
answer has a name:
pi(x) = the number of primes less than or equal to x.
The primes under 25 are 2, 3, 5, 7, 11, 13, 17, 19 and 23 so pi(3) = 2, pi(10) =
4 and pi(25) = 9.