stressor

noun

1. An environmental condition or influence that stresses (i.e. causes stress for) an organism.

   “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.”