There is no federal law qualifying who a domestic partner includes, though some state and local laws do. If your company intends to cover domestic partners, you should outline what a domestic partner is in your employee handbook. Some common elements of domestic partnership can include:
Unmarried individuals in partnership
Minimum age of 18 for both parties
Both are mentally competent and able to enter into a contract in residing state
Neither are family members of each other through blood relation
Both must share a committed, exclusive relationship with each other
Both must be financially independent
Non-employee must not qualify for benefits through his/her own employer
Jointly responsible for financial decisions
Reside in the same residence for a minimum amount of time
Meet the requirements for specific plans
Both must sign employee affidavit to confirm spousal equivalent