stressor

1. (psychology, biology) An environmental condition or influence that stresses (i.e. causes stress for) an organism.Examples: "These children are constitutionally sad. Other children, like Luke, develop depressive feelings out of the blue or in response to some mild stressor."; "These paradoxical pleasures […] are acquired by controlling one's exposure to the stressor in gradually increasing doses."; "Plenty of people see hosting a large group as a stressor."