A more thorough explanation:

A stepchild is a child who is the biological or legally adopted child of your spouse from a previous relationship. If you have not legally adopted the child, they are still considered your stepchild. However, if you do legally adopt the child, they become your legal child and have the same rights as if they were biologically related to you.

• If your spouse has a child from a previous marriage, that child is your stepchild.
• If you legally adopt your stepchild, they become your legal child.

The relationship between a stepchild and stepparent has different implications in different areas of law. For example:

If you leave property to your children in your will, state law will determine whether a stepchild qualifies as a child when used generally in a will and whether it means only biological children.

In order for a stepchild to be able to immigrate as a "child," the marriage creating the stepchild/stepparent relationship must have happened before the stepchild was 18 years of age.

If you want to claim your stepchild as a dependent on your tax returns, the IRS states that your stepchild must live with you for more than six months of the calendar year, and the child has to be younger than you are unless the child has a permanent disability.