Estrangement is the feeling that you don't belong, especially when you're surrounded by other people. If you've ever had to sit by yourself in a school cafeteria, you know what estrangement feels like.
Right in the middle of the word estrangement is strange, and it provides a great clue about how to use the word. Estrangement happens when something — or someone — makes you feel like a stranger. It can describe a couple that's split up or the alienation of a former friend. The meaning hasn't changed much from its Latin root, extraneare, "treat as a stranger."