In Euclid's Elements, Book VII, Definition 11, Euclid states that
A prime number is that which is measured by a unit alone.
As I understand it, this means that a prime number's only divisor is 1. Obviously the modern definition is that a prime number's only divisors are 1 and itself.
This is a semantic difference I know, but what part about Euclid's definition am I misunderstanding?