Wiktionary
n. The parent (either of a separated pair) with whom a child or children do not live the majority of the time.
Wikipedia
A noncustodial parent is a parent who does not have physical and/or legal custody of his/her child by court order. << you can be considered a non-custodial parent even while having shared physical and/or legal custody.
A "child-custody determination" means a judgment, decree, or other order of a court providing for the legal custody, physical custody, or visitation with respect to a child. The term includes a permanent, temporary, initial, and modification order. The term does not include an order relating to child support or other monetary obligation of an individual. Where the child will only live with one of the parents, sole physical custody is ordered, and the parent with which the child lives is the custodial parent, the other parent is the non-custodial parent.
Note, however, where the child will live with both parents, joint physical custody is ordered, and both parents are custodial parents. In the case of joint physical custody a 50/50 equal shared parenting schedule is typically not required, therefore the joint custodial parent may have the minority of time with the child but not be said to be a non-custodial parent. For example, states such as Alabama, California, and Texas do not necessarily require joint custody orders to result in substantially equal parenting time, whereas states such as Arizona, Georgia, and Louisiana do require joint custody orders to result in substantially equal parenting time where feasible.