Stress is 'the adverse reaction people have to excessive pressure or other types of demand placed on them'. Pressure is part and parcel of all work and helps to keep us motivated. But excessive pressure can lead to stress which undermines performance, is costly to employers and can make people ill. One of the founding fathers of stress research, Hans Selye, identified another part of this problem when he saw that different types of definition operate in different areas of knowledge. To a lawyer or linguist, words have very precise, definite and fixed meanings. In other fields, ideas and definitions continue evolving as research and knowledge expand.