Offspring of parentage; progeny.
Source: U.S. Courts
Generally, an unmarried person under 21 years of age who is: a child born in wedlock; a stepchild, provided that the child was under 18 years of age at the time that the marriage creating the stepchild relationship occurred; a legitimated child, provided that the child was legitimated while in the legal custody of the legitimating parent; a child born out of wedlock, when a benefit is sought on the basis of its relationship with its mother, or to its father if the father has or had a bona fide relationship with the child; a child adopted while under 16 years of age who has resided since adoption in the legal custody of the adopting parents for at least 2 years; or an orphan, under 16 years of age, who has been adopted abroad by a U.S. citizen or has an immediate-relative visa petition submitted in his/her behalf and is coming to the United States for adoption by a U.S. citizen.
Source: U.S. Citizenship and Immigration Services
Unmarried child under the age of 21 years. A child may be natural born, step or adopted. If the child is a stepchild, the marriage between the parent and the American citizen must have occurred when the child was under the age of 18. If the child is adopted, he/she must have been adopted with a full and final adoption when the child was under the age of 16, and the child must have lived with and been in the legal custody of the parent for at least two years. An orphan may qualify as a child if he/she has been adopted abroad by an American citizen or if the American citizen parent has filed an immediate-relative (IR) visa petition for him/her to go to the United States for adoption by the American citizen.
In certain visa cases a child continues to be classified as a child after he/she becomes 21, if the petition was filed for him/her when he/she was still under 21 years of age. For example, an IR-2 child of an American citizen remains a child after the age of 21 if a petition was filed for him/her on or after August 6, 2002, when he/she was still under 21 years old. The child must meet other requirements of a child as listed above.
Source: U.S. Department of State.