olemus
ÕS: ajend, virgutus, tõuge

Wiktionary:
1. 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
2. (physiology, psychology, medicine) something external that elicits or influences a physiological or psychological activity or response, or that affects any of the sensory apparatuses

ülevaateid
https://en.wikipedia.org/wiki/Stimulus_(economics)
https://en.wikipedia.org/wiki/Stimulus_(physiology)