A member of household is defined by state and federal laws, which varies by jurisdiction. The Internal Revenue Service (IRS) defines a member of household as a person who is related to you or lives with you for the entire year as a member of your household.
The IRS explains a member of household for tax purposes as follows:
"If the person is not related to you, he or she must have lived in your home as a member of your household for the entire year (except for temporary absences, such as for vacation or school). A person is not a member of your household if at any time during your tax year the relationship between you and that person violates local law."