A mass budget is the strict limit on how much weight a spacecraft, aircraft, drone, or other platform can carry. Every component must fit inside that limit, so designers trade off sensors, batteries, shielding, compute hardware, cabling, and structural parts against flight range, stability, and reliability.
In cyber security, mass budget matters for embedded and autonomous systems because security controls are not free. Extra encryption hardware, tamper protection, logging storage, or backup radios all add weight, and a tight budget can force difficult choices. Attackers also care about this constraint: on small vehicles, they may prefer lightweight implants, hidden transmitters, or malicious hardware that is hard to notice because there is little spare capacity. Defenders respond by designing security features that are compact, validating that added protection does not break mission performance, and planning fallback modes when resources run out.