The custodial parent is the parent with primary custody of a child. In other words, a custodial parent is a child's main caregiver.
A custodial parent is considered the legal parent of a child. If the child has another legal parent, they are considered the child's non-custodial parent.
To become a child's custodial parent, a judge must give the parent primary custody in court. This court order is called a parenting plan.
Two parents that have joint-custody of a child would both be the child's custodial parent.