Vacuum
Vacuum is an environment with gas pressure far below ordinary atmospheric pressure; space is an extremely low-density environment rather than a perfect absence of matter.
Technical meaning
Engineering use requires more than the label: teams specify the applicable pressure, reference conditions, time interval, interfaces, and evidence. The same term can be implemented differently across missions, but the underlying distinction must remain explicit and traceable.
Why it matters
Vacuum conditions influence material selection, lubrication, thermal behavior, outgassing, propulsion, testing, crew protection, and spacecraft lifetime. Clear use of the term reduces interface errors and makes engineering tradeoffs easier to audit.