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By default, Microsoft Office Project defines a task (task: An activity that has a beginning and an end. Project plans are made up of tasks.) as critical (critical task: A task that must be completed on schedule for the project to finish on time. If a critical task is delayed, the project completion date might also be delayed. A series of critical tasks makes up a project's critical path.) if it has zero days of slack (slack: The amount of time that a task can slip before it affects another task or the project's finish date. Free slack is how much a task can slip before it delays another task. Total slack is how much a task can slip before it delays the project.). However, you can change this definition and make a task critical if it has, for example, one or two days of slack.