Definition of Prime number. What are prime numbers?
This concept is sometimes very confusing for some people. A very common definition for a prime number is a number which has only two factors.

Ok then what about 3? Is 3 a prime number or not. In the first instance you would say that it is a prime number, but how can you prove it. I am going to show you something really interesting.
1×3=3
-1x-3=3
Factors for 3 are 1,3,-1,-3
That means 3 is not a prime number

What the hell happened?

Were we learning the wrong thing for our entire life?

No guys you guys were right. I would just like to add something in to the definition. Prime number is a number which has 2 positive factors. So the positive factors of 3 are 1 and 3. In order words you can say that a prime number is a number which has only 2 positive divisors.