Per IRS Publication 17, Your Federal Income Tax For Individuals, on page 21:

Considered married. You are considered married for the whole year if, on the last day of your tax year, you and your spouse meet any one of the following tests.

1. You are married and living together.
2. You are living together in a common law marriage recognized in the state where you now live or in the state where the common law marriage began.
3. You are married and living apart, but not legally separated under a decree of divorce or separate maintenance.
4. . You are separated under an interlocutory (not final) decree of divorce.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.