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“Human factors [and ergonomics] is the study of how humans behave physically and psychologically in relation to particular environments, products, or services” (SearchSOA.com).
Human factors is an umbrella term for several areas of research that include human performance, technology design, and human-computer interaction. The study of human factors in the Human Factors Research and Technology Division at NASA Ames Research Center focuses on the need for safe, efficient and cost-effective operations, maintenance and training, both in flight and on the ground.- As defined by NASA
More human factors definitions can be found here.