A squarefree integer is one not divisible by the square of any prime. Equivalently, every prime factor appears with exponent exactly 1. Examples: 1, 2, 3, 5, 6 (= 2·3), 7, 10 (= 2·5), 11, 13, 14, 15, 17, 19, 21, 22.
Non-examples: 4 (= 2²), 8 (= 2³), 9 (= 3²), 12 (= 2² · 3), 18 (= 2 · 3²), 25 (= 5²).
Squarefree numbers have density \(6/\pi^2 \approx 0.6079\) in the natural numbers — roughly 61% of integers are squarefree. The Möbius function \(\mu(n)\) is zero precisely on non-squarefree numbers, and \(\pm 1\) otherwise.