Webster defines

**prime** (prim) *n*.[**ME**, fr. **MF**, fem. of *prin*
first, **L** *primus*; akin to **L** *prior*] **1**
:first in time: **original 2 a** : having no factor except itself and one (3
is a number) **b** : having no common factor except one ( 12 and
25 are relatively ) **3 a** : first in rank, authority or significance
: **principal b** : having the highest quality or value ( television time)
[from *Webster's New Collegiate Dictionary*]

A prime number is any number with no divisors other than itself and 1, such as 2 and 11. Any number can be written as a product of prime numbers in a unique way (except for the order).