An 'issue' is supposed to be a clear, well-defined task that can be tested. When creating an issue a storypoint value is estimated and a team member is assigned to the task. We manage and monitor issues in JIRA system. Plenty of subgenres exist: well-defined issue, issue with a SP value, urgent issue, no-one's issue, still-needs-work issue.
Issues are also notable for their presence in our everyday language:
You don't like something? Create an issue for that.