In agile methodologies, the term "product backlog" refers to a prioritized list of work items or features that need to be completed for a product. It serves as the single source of requirements for the development team, facilitating a transparent view of the project's progress. Each item in the backlog is expressed as user stories, providing context for developers and stakeholders. The product backlog is continuously refined and re-prioritized based on feedback and changing goals, ensuring the team focuses on delivering maximum value in sprints.