The terms human factors and ergonomics have been defined in many ways over the years, but this definition from Computer Ergonomics for Elementary School (CergoS) captures the essence of the field in a concise way:
Ergonomics and human factors use knowledge of human abilities and limitations to design systems, organizations, jobs, machines, tools, and consumer products for safe, efficient, and comfortable human use.
Source: http://osha.oregon.gov/edu/grants/wrd/cergos/Pages/ergonomics.aspx. Used with permission.
The following definition was adopted by the International Ergonomics Association in August 2000:
Ergonomics (or human factors) is the scientific discipline concerned with the understanding of interactions among humans and other elements of a system, and the profession that applies theory, principles, data, and other methods to design in order to optimize human well-being and overall system performance.
Numerous other terms and definitions may be found in the Educational/Professional section of the site.
What Is Human Factors/Ergonomics?