Erstwhile means former or previous. When you describe someone as your erstwhile friend, you mean they used to be your friend but aren't anymore. An erstwhile champion is someone who was once a champion but has since lost that title.

The word carries a slightly formal, almost old-fashioned tone. You might read in a history book about an erstwhile colony that gained independence, or hear about an erstwhile rivalry between two schools that has since been forgotten. A teacher might refer to herself as an erstwhile student at the school where she now works.

Unlike “ex-” or “former,” which sound neutral, erstwhile can suggest a touch of nostalgia or significance about what used to be. It's particularly useful when writing about historical changes or transformations. You're more likely to encounter erstwhile in books or formal writing than in everyday conversation, where people usually just say “former” or “used to be.”