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The margin allocated during design based on assessments of uncertainty and unknowns. This margin is often consumed as the design matures.
NIST SP 800-160v1r1 from NASA Systems Engineering Handbook
A spare amount or measure or degree allowed or given for contingencies or special situations. The allowances carried to account for uncertainties and risks.
NIST SP 800-160v1r1 under margin from MTR210263