First that term refers to integers.
So the statement that is divisible by means that each of are integers such that
That means that cannot be divisible by . Do you see why?
That definition also is used in defining prime numbers: an integer is prime if and only if it has exactly two divisors.
Often even well respected textbooks and commentators (i.e. Keith Devlin the math guy) say that an integer is prime if "it is only divisible by itself and one". But that definition makes one a prime. Do you see why?
BTW: (Thanks to R L Moore for that distinction,)