The ordered list of everything the team might build: features, fixes, technical work, and ideas. Items at the top are described in detail and ready to start; items further down are deliberately rough.
A backlog is where priorities become visible and arguable. Without one, work arrives through side channels, urgent beats important, and nobody can say what was traded away to fit something in.
We keep one backlog per product, groomed before each planning session. Every item carries a short business reason, so when scope must be cut, the conversation is about outcomes rather than tickets.