This term is defined in ECSS-E-ST-10C as
characteristic of a process, process condition, parameter, requirement or item that deserves control and special attention in order to meet the objectives (e.g. of a mission) within given constraints

This term is defined in ECSS-S-ST-00-01C as
resulting in temporarily disabling but not life threatening injury, temporary occupational illness, major detrimental environmental effects, major damage to public or private properties, major damage to interfacing flight systems or major damage to ground facilities