Final answer:
The 'Birthday Rule' is a method used by health insurance companies to decide which parent's insurance is primary for a child when both parents have coverage.
Step-by-step explanation:
The term you're referring to is the Birthday Rule.
This rule is used in the health insurance industry to determine which parent's health insurance plan will be the primary coverage for a dependent child when both parents have coverage.
Essentially, the parent whose birthday (month and day, not year) occurs first in a calendar year will have their health insurance plan billed first for the child's healthcare expenses.