The margin allocated during design based on assessments of uncertainty and unknowns. This margin is often consumed as the design matures.
Sources:
NIST SP 800-160v1r1
under design margin
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.
Sources:
NIST SP 800-160v1r1
from
MTR210263
The margin that is designed explicitly to provide space between the worst normal operating condition and the point at which failure occurs (derives from physical design margin).
Sources:
NIST SP 800-160v1r1
under operational margin
from
Systems Engineering and System Definitions