Required by circumstances; absolutely needed.

Military necessity, along with distinction, and proportionality, are three important principles of international humanitarian law governing the legal use of force in an armed conflict.

a legal doctrine in the United States related to activities that may be justified as reasonable, necessary, and/or appropriate based on evidence-based clinical standards of care.

In philosophy, metaphysical necessity, sometimes called broad logical necessity,Brian Leftow, God and Necessity.

Justification due to unavoidable circumstances.