The Working Families Tax Relief act of 2004 has redefined "dependent." There are now two categories.
Qualifying child
Qualifying relative
A qualifying child is:
Your son, daughter, stepchild, adopted child, foster child, brother, sister, stepbrother, stepsister, or a descendant of any of them who:
Was under age 19 at the end of 2007 or under age 24 and a full-time student or any age if permanently and totally disabled.
Did not provide over half of his/her own support.
Lived with you more than half of 2007.
A qualifying relative is:
Related to you or lived with you for the full year and who:
Is not a qualifying child of any other person in 2007.
Had gross income less than $3,400.
Is a person for whom you provided over half of his/her support for 2007.
The qualifying child or qualifying relative must also:
Be a U.S. citizen or national, or a resident of the United State, Canada, or Mexico.
Not file a joint return with his or her spouse unless the return is filed only to claim a refund and no tax liability would exist for either spouse if separate returns were filed.
You cannot claim a dependent if you can be claimed as a dependent by another person.