A Blocker is an impediment that completely prevents further work on an item. Unlike a delay, where the team progresses more slowly, a Blocker stops progress entirely. This is why resolving Blockers has the highest priority: every blocked item increases Work in Progress without any value being created.
In practice, a Blocker is named during the Daily Scrum or stand-up and visually marked on the board, for instance with a red flag or a dedicated Blocker column. A typical example: the team is waiting for API access from an external partner and cannot continue building the integration. The Scrum Master or the team takes on the resolution immediately rather than hoping the problem will resolve itself. Some teams maintain a Blocker log to identify recurring patterns and address structural root causes.
The term appears in both Scrum, where it is known as an Impediment, and in Kanban. In Kanban systems, Blockers make especially clear why WIP Limits matter: without limits, teams simply start new work instead of resolving the Blocker.
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