A prime number is a number that can be divided by only itself and 1.
For example,

2 is a prime number. It can only be divided exactly by 1 and 2 itself.

3 is a prime number. It can only be divided exactly by 1 and 3 itself.

4 is not a prime number. It can be divided exactly by 1, 2 and 4.

Dictionary Definition

The Oxford English Dictionary defines a prime number as "a number that cannot be divided by any whole number (without a remainder) other than itself and one."

The Prime Numbers

The prime numbers are:

In a number square, the prime numbers are shaded below:

These are just the prime numbers under 100. There are infinitely many prime numbers, they go on forever.

Prime Numbers Are Natural Numbers Greater Than 1

Prime numbers are natural numbers (the counting numbers: 1, 2, 3...) greater than 1.
1 is not a prime number, even though it can only be divided by 1 and itself. Some times in history is has been considered a prime number, but now it is not.

Interactive Game on Prime Numbers

Here is an interactive game to help you learn about prime numbers.

You are the fish.

Pop the bubbles with prime numbers to collect red hearts. You need five to win.

If a bubble with an prime number reaches the top, you will lose a red heart.

Avoid the bubbles with composite numbers (i.e., non-prime numbers). If you pop one, you will lose one of your three lives.

Good luck!

The fish moves to wherever you taptowards your mouse clicks. (Hint: The farther the distance, the faster it moves.)

Factors

Numbers that divide exactly into another number are called factors.
For example, the factors of 4 are 1, 2 and 4 because they all divide exactly into 4.
Prime numbers only have two factors, 1 and the prime number itself.
The only factors of 2 are 1 and 2. The only factors of 3 are 1 and 3. The only factors of 5 are 1 and 5.