That which is required for continued existence or function.
Blood that is needed for continued life; blood regarded as the seat of life.
Something external that elicits or influences a physiological or psychological activity or response, or that affects any of the sensory apparatuses.
A sting on the body of a plant or insect.
An external phenomenon that has an influence on a system, by triggering or modifying an internal phenomenon; for example, a spur or incentive that drives a person to take action or change behaviour.