What is Alternating Custody?

Legal Definition
Alternating custody, also known as serial custody or shared custody, is an arrangement in which children live for long periods of time with one parent, and then spend a similar amount of time with the other parent. The primary difference between alternating custody and joint custody is that in alternating custody, the parent that currently has the child also retains sole authority over the child/children for the duration that the child/children are with said parent. The general reason for using this arrangement rather than a more commonly used arrangement is that the parents tend to live too far away from each other to allow for other arrangements to be feasible.
-- Wikipedia
Legal Definition
the division of custody of a child or children that is between the 2 separated or divorced parents. See divided custody.