Issue
A matter that is in dispute between two or more parties or a point of conflict which may prevent a project or service from meeting its objectives. Issues are distinct from risks (which might happen) in that issues are happening.
Impact
Unresolved issues block progress. The "Issue Log" is the project manager's primary tool for clearing the path.
Weinto take
We prefer the Agile term "Blocker." An issue sounds like something to discuss; a blocker sounds like something to destroy. We encourage a culture where raising a blocker is seen as a helpful signal, not a complaint.