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Words using the root word “stress” can be confusing. In 1936, Selye, a medical researcher, described what is now called stress—the biological responses (effect) seen after exposure to a harmful agent (cause). Initially, the word “stress” was used for both the agent and the response. In 1975, Selye proposed calling the agent “stressor” and the response “stress.” Not all dictionaries and researchers have adopted Selye’s terms. This can explain the confusion about the use and meaning of “stressful.”
For more information, but not guanteed to make it any clearer), see: https://www.thefreedictionary.com/stress (noun definition 6) https://www.thefreedictionary.com/stressor https://www.thefreedictionary.com/stressful
Typical mistake of vietnamese students learning English. They use 'stressful' when they mean 'stressed'