Child. A child is usually defined as a direct descendant of the dececent. That means child, grandchild, greatgrandchild and so on. Legally adopted children are treated just like lineal descendants, so they count, too. And that means that once a child is legally adopted by another, that child's legal ties to the birthparent are legally severed, which means that they don't count for inheritance purposes. Children who were born after a parent dies count as children for inheritance purposes.
Stepchildren who were never legally adopted don't usually count as children for intestate purposes. Stepchildren who were adopted by a stepparent can still inherit from their biological parent, but this is dependent on state law. In some states, an unadopted stepchild may qualify as an heir if certain conditions are satisfied, such as that the relationship with the parent started while the stepchild was a minor and continued throughout the parent's lifetime and the parent would have adopted that child but there was some legal barrier to doing so (like the parent's natural parent refusing to consent to such adoption).