The most commonly accepted definition of stress (mainly attributed to Richard S Lazarus) is that stress is a condition or feeling experienced when a person perceives that “demands exceed the personal and social resources the individual is able to mobilize.” In short, it's what we feel when we think we've lost control of events.
http://www.mindtools.com/pages/article/newTCS_00.htm
The UK Healthy and Safety Executive defines stress in the following way: "the adverse reaction people have to excessive pressure or other types of demand placed on them".
Lazarus (1966) offers this definition of stress: "stress occurs when an individual perceives that the demands of an external situation are beyond his or her perceived ability to cope with them". (pg 9)
http://www.hypnos.info/book2/page2.html
Stress is a reaction to continued excessive pressure or responsibility when you feel inadequate or unable to cope.
http://www.stressbusting.co.uk/workplace-stress/what-is-stress-definition-of-stress/
Stress arises when individuals perceive that they cannot adequately cope with the demands being made on them or with threats to their well-being. R.S. Lazarus (1966). Psychological stress and the coping process. New York: McGraw-Hill.
Stress, it is argued, can only be sensibly defined as a perceptual phenomenon arising from a comparison between the demand on the person and his or her ability to cope. An imbalance in this mechanism, when coping is important, gives rise to the experience of stress, and to the stress response. T. Cox (1978). Stress. Basingstoke: Macmillan Education.
Stress results from an imbalance between demands and resources. R.S. Lazarus and S. Folkman (1984). Stress, Appraisal and Coping. New York: Springer.
Stress is the psychological, physiological and behavioural response by an individual when they perceive a lack of equilibrium between the demands placed upon them and their ability to meet those demands,