Glossary
prime number
A prime number has exactly two factors: 1 and itself. 2, 3, 5, 7 and 11 are prime; 1 is not, and 9 is not because 3 divides it.
A prime number is a whole number greater than 1 whose only factors are 1 and itself. 2, 3, 5, 7, 11, 13, 17, 19 and 23 are the primes below 25. The definition sounds trivial and does most of the work in a question: checking whether a number is prime means testing whether anything else divides it.
Two special cases turn up. 1 is not prime, because it has only one factor rather than two. 2 is the only even prime, since every other even number is divisible by 2 as well as by itself; that makes 2 the exception to the useful shortcut that a number is not prime if it ends in an even digit or a 5 (other than 5 itself).
To test a number, divide by the primes up to its square root; if none divides evenly, the number is prime. Nothing under 100 needs more than 2, 3, 5 and 7. Primes matter because they are the building blocks of the whole numbers — every number is prime, or a product of primes — which is why questions ask you to factor numbers and to find the greatest common factor or the least common multiple of two of them.
Taught in
Related terms
- greatest common factor — The greatest common factor of two numbers is the largest number that divides both evenly. It is used to simplify a fraction to lowest terms.
- least common multiple — The least common multiple of two numbers is the smallest number both divide into evenly. It is the common denominator when you add fractions.
- integer — An integer is a whole number — positive, negative or zero: …, −2, −1, 0, 1, 2, …. Nothing with a fractional part is one, so 3.5 is not an integer and neither is 1/2.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)