A composite number is a number that can be divided exactly by at least one number that is not itself or 1.
This means a composite number is any number (other that 1) that is not a prime number.
For example,

4 is a composite number. It can be divided exactly by 1, 2 and 4.
4 is a composite number because it can be divided by 2 (which is not 1 or 4 itself).

5 is not a composite number. It can only be divided exactly by 1 and 5.
5 is a prime number because it can only be divided by 1 and 5 itself.

Dictionary Definition

The Oxford English Dictionary defines a composite number as "a number which is the product of two or more factors, greater than unity."

The Composite Numbers

The composite numbers are:

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

Note: This is the exact inverse of the prime numbers in a square. If this number square is overlaid with that of the prime numbers, all numbers (apart from 1) would shaded.

Composite Numbers Are Natural Numbers Greater Than 1

Composite numbers are natural numbers (the counting numbers: 1, 2, 3...) greater than 1.

Interactive Game on Composite Numbers

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

You are the fish.

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

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

Avoid the bubbles with prime numbers (i.e., non-composite 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.
Composite numbers must have more than 2 factors: 1, the composite number itself, and at least one other factor.
This is in distinction to prime numbers which only have two factors: 1 and the prime number itself.