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A Task is a specific defined piece of work that, combined with other identified Tasks, composes the work in a specific specialty area or work role.
NIST SP 800-181 under Tasks
Activities required to achieve a goal. Note that activities can be physical or cognitive.
NISTIR 8040 under Task
Required, recommended, or permissible action, intended to contribute to the achievement of one or more outcomes of a process.
NIST SP 800-160 [Superseded]