Per IRS Publication 17 Your Federal Income Tax For Individuals, on page 27:

Qualifying Child

Five tests must be met for a child to be your qualifying child. The five tests are:

1. Relationship,
2. Age,
3. Residency,
4. Support, and
5. Joint return.

CAUTION! If a child meets the five tests to be the qualifying child of more than one person, there are rules you must use to determine which person can actually treat the child as a qualifying child. See Qualifying Child of More Than One Person, later.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.