Coprime
https://en.wikipedia.org/wiki/Coprime_integers
In number theory, two integers a and b are said to be relatively prime, mutually prime, or coprime if the only positive integer (factor) that divides both of them is 1. Consequently, any prime number that divides one of a or b does not divide the other. This is equivalent to their greatest common divisor (gcd) being 1.
Incorrect answers:
Convergent - a series is the sum of the terms of an infinite sequence of numbers.
Euler's totient function - counts the positive integers up to a given integer n that are relatively prime to n. It is written using the Greek letter phi as φ(n) or ϕ(n), and may also be called Euler's phi function. In other words, it is the number of integers k in the range 1 ≤ k ≤ n for which the greatest common divisor gcd(n, k) is equal to 1. The integers k of this form are sometimes referred to as totatives of n.
Fermat's number - named after Pierre de Fermat, who first studied them, is a positive integer of the form where n is a non-negative integer.