Primes or prime numbers are integers greater than 1 which are divisible only by themselves and 1, i.e.: 2, 3, 5, 7, 11, ..
A square root of a number A is a number y such that y^2 = A, or, in other words, a number y whose square (the result of multiplying the number by itself, or y × y) is A.
"Iterate over the primes already found which are less than the square-root of p"
from question "Why does this naive prime-number algorithm fail?"