Define child marriage, including its implications.

Step 1: Define "child marriage".
Child marriage refers to any formal or informal union where one or both parties are under the age of 18. It is a human rights violation that deprives children, particularly girls, of their childhood, education, and health, and exposes them to violence and exploitation.

The definition of "child marriage" is:

$$\boxed{\text{Child marriage is a formal or informal union where one or both parties are under the age of 18.}}$$