What is IRS' definition of "spouse"?
In my culture it is not that common to get married, but we do have life partners. I have been living with my partner for six years. Are we considered common law married? Can we file as spouses?

Misslead:

If your local government considers you legally married, then the IRS considers you legally married.

Until about 1997, common law marriage was recognised in Georgia. I had several clients who were common-law married and they never had a problem.

Thank you for your helpful reply :)

Do you know from who or where to get the information concerning how the individual state defines married?

A call to your local city/county courthouse will probably get you an answer as to how state law addresses the marriage issue.