A marker introduced in Terraform 1.10 that indicates a value (variable, output, or resource result) must never be persisted to state or plan, useful for temporary credentials or other sensitive data that shouldn't be stored on disk.
In Terraform Associate it's compared to sensitive=true: sensitive hides the value in console logs but DOES save it in state; ephemeral=true simply never persists it anywhere, offering greater security for short-lived secrets.
Ephemeral values is a concept that can show up on the exam. Check the full guide to see it in context.