Per IRS Publication 501 Dependents, Standard Deduction, and Filing Information, on page 10:
The term "dependent" means:
On page 11:
Tests to be a Qualifying Relative
See IRS Publication 501 for exceptions to the qualifying relative tests.
For more information on qualifying children, see Dependents - Rules For Claiming. For instructions on how to enter dependent information in TaxAct, see Dependents - Adding in Program.
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.