Sentence:  "I have a patient right now whose <WSD>marriage</WSD> proved to be a tragedy. She wanted love, sexual gratification, children, and social prestige; but life blasted all her hopes. Her husband didn't love her. He refused even to eat with her, and forced her to serve his meals in his room upstairs. She had no children, no social standing. She went insane; and, in her imagination, she divorced her husband and resumed her maiden name. She now believes she has married into the English aristocracy, and she insists on being called Lady Smith. 
Meaning of the word marked with <WSD> tags:  The union of only two people, to the exclusion of all others.