The possibly emotional, legal, financial, and/or religious partnership between two (or more, if you're open-minded) individuals. While this encompasses the definition of marriage, it does not in any way sum up marriage itself.
The state of matrimony can best be described subjectively. It can be worse than "Marriage is when two people love each other so much that they want to spend the rest of their lives driving each other crazy." and better than wanting to spend the rest of your life with someone just to make them happy.
It occurs to me that these could be good things to keep in mind for any relationship.