Prime Counting Function -- from Wolfram MathWorld

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The prime counting function is the function pi(x) giving the number of primes less than or equal to a given number x (Shanks 1993, p. 15). For example, there are no primes <=1, so pi(1)=0. There is a single prime (2) <=2, so pi(2)=1. There are two primes (2 and 3) <=3, so pi(3)=2. And so on. The notation pi(n) for the prime counting function is slightly unfortunate because it has nothing whatsoever to do with the constant pi=3.1415. This notation was introduced by number theorist