Stress has no precise definition when considered biologically.
In popular usage, stress is understood to describe the feelings people get when they consciously feel that things are getting "too much for them".
Stress is often defined as "emotional or mental pressure" in general dictionaries, and yet when people say they are stressed, it can be physical stresses in the form of having too much work, or emotional stresses in the form of difficulties interacting with others, having problems with health, or not being able to cope with their current life issues. - Quite a broad range!
From Psychology, one of the many definitions offered which I like is that which regards stress as being the body's response to demands made upon it, mediated largely by the autonomic nervous, and hormonal systems which are within our subconscious brain regions.
Since the feelings that characterize anxiety or fear also reflect the bodily changes brought about by the autonomic nervous, and hormonal systems, I personally see only subjective intensity as the difference between the terms stress, anxiety and fear.
What Causes Stress?
From the broader biological view, the body can be stressed by over-exertion in many forms, and this itself can also lead to anxiety. The anxiety loop that this can cause is referred to below - but this page is more concerned with the psychological view of stress.
The diagram above does not indicate any direct causes of stress, it simply shows some of the physical, mental, emotional, and behavioural symptoms of stress.
This is not surprising as the question of causes cannot be answered with certainty - there are too many views on the subject and an abundance of possible answers.
When we feel that things are weighing down on us, we often say that we are stressed out. Physical and/or mental pressure may directly give rise to such feelings - but possibly indirectly too.
We may fear that we will not be able to meet the expectations of others, our accepted social obligations, our deeply held beliefs about how we should perform etc - and so underlying the actual source of "stress" may be the fear of failure, or of being found out.
Stress may be precipitated by an underlying fear related to actions which we have wither taken, are thinking of taking, or just to situations in which we happen to find ourselves, but feel unable to control.
Another example - we may become consciously taxed by the burden of something that is not working out. This may lead us to begin losing self confidence or self esteem, and come to believe that failure is inevitable. Negative thoughts can induce stress reactions.
Our brain monitors our conscious worries and concerns and if it perceives a risk of us violating our current belief system, it is likely to initiate a fear response - the intensity of which may be variable.
Stress feeds in to anxiety
Stress is most problematic to us when it persists. Then the body continues trying to respond to the demands being made upon it to support resolution of the issue, by sustaining the extra resources provided by our autonomic nervous, and hormonal systems.
Prolonged activation of these systems stresses the physical body further, and can lead over time to intensifying the physical sensations like rapid heartbeat, physical agitation, stomach upsets etc.
These symptoms are felt physically and cause us concern, and this concern feeds back into the loop - causing the autonomic nervous and hormonal systems to try and provide even more resources to bolster the body!
This is the "anxiety loop" and is mentioned further on the anxiety tab.
If you feel that you have issues which cause you persistent anxiety, stress or fear and you are unable to improve matters alone, then please consider talking to your GP or a local qualified therapist for guidance and help.