essential function

This term is defined in ECSS-E-ST-20C as
function without which the operator cannot recover the spacecraft (following any conceivable on-board or ground-based failure), the spacecraft cannot be commanded, the spacecraft permanently loses attitude and orbit control, the spacecraft consumables (e.g. fuel and energy) are depleted to such an extent that more than 10% of its lifetime is affected, or the safety of the crew is threatened