Primary custody – this term is used in the area of Family Law. There it relates to the figure of the parent, to whom the child is left to live with (with a court decision usually), after the divorce. This is why sometimes it the primary custody is called also “physical custody”. The opposite term is “joint custody”, which details a court decision on divorce trials, where it is proclaimed that both parents will exercise the custody of their child(ren).

See also http://singleparents.about.com/od/childcustodyresources/a/primary_custodian.htm