Simple Definition:

An even number is a whole number that can be divided by 2 without leaving a remainder. This means that when you divide it by 2, you get another whole number. [Whole number means a number without fractions or decimals.]

Examples: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20.

Very Simple Definition:

An even number is a number that can be split into two equal groups.

Examples: If you have 4 cookies and you want to share them equally with your friend, both of you will get 2 cookies each. 4 is an even number. Another example is if you have 6 crayons and you want to divide them equally between you and your sibling, both of you will get 3 crayons each. 6 is also an even number.