Human Factors is the science of work as its Greek translation implies, ergon meaning work and nomos meaning laws. There are a number of definitions which go along way to explain what exactly the discipline entails. Two of these definitions are listed below.
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 methods to design in order to optimize human well-being and overall system performance. International Ergonomics Association
Human Factors is concerned to optimize the relationship between people and their activities, by the systematic application of human sciences, integrated within the framework of systems engineering. Professor Elwyn Edwards cited in ICAOs CAP 719, Fundamental Human Factors Concepts
Human Factors contains a vast amount of varied disciplines. For the aviation sector we can brake it down into three domains. Each of these domains contributes to the overall performance of the human in the system with no one domain having a greater influence than the others. In order for the human performer to contribute at their best, all of these domains need to be addressed.
The physical domain is concerned with human anatomical, and some of the anthropometric, physiological and biomechanical characteristics as they relate to physical activity
The cognitive domain is concerned with mental processes, such as perception, memory, reasoning, and motor response, as they affect interactions among humans and other elements of a system.
The organisational domain is concerned with the optimisation of socio technical systems, including their organizational structures, policies, and processes.