There are as many variations of this as there are jurisdictions.
In Canada, the Child Support Guidelines defines shared custody (which then allows for a unique child support formula), as:
"... Where a spouse exercises a right of access to, or has physical custody of, a child for not less than 40 per cent of the time over the course of a year..."
Some American states call this joint physical custody or shared parenting.
Relying on Kansas family law, Justice Rulan of the Court of Appeals of Kansas wrote, in Re the Marriage of Dwayne Roth:
"... shared custody is a more specific type of joint custody. Joint custody is a legal designation, with both parents having equal rights to make decisions in the best interest of the child.
"Joint custody may be of two physical types: (1) shared custody, where the child's residence is divided in an equal manner with regard to time of residency or (2) primary residency, where the child's residence is on the basis of a primary residency arrangement."
In V.B. v M.L.T.B., and again relying heavily on a statutory definition, Justice Lipez wrote this in the footnotes to the opinion of the Supreme Court of Pennsylvania:
"The definition of shared custody is an order awarding shared legal or shared physical custody or both of a child in such a way as to assure the child of frequent and continuing contact, including physical access, to both parents.
"It is clear that frequent and continuing contact may be assured by many custody arrangements other than that which gives each parent an equal number of days or hours with the child."