Now, I can understand why you clicked on this link, which was headed 'what is stress?'. It isn't that you're stupid or that your English vocabulary is poor, or that you're puzzled by the concept. You probably just don't want to miss a page, or want to check that our definition matches what you already believe or...
The fact is that you don't really need to read this page, and it is kind of sad that the paragraph in blue below is pretty much compulsory in discussions of teacher stress.
What is stress? Stress is not an illness, though it frequently contributes directly to mental and physical illnesses and is linked closely with anxiety and depression.
The key point is this: IT DOESN'T MATTER. It really doesn't matter whether stress is an illness or not; to the layman, the symptoms look pretty much like the symptoms of an illness; and the distinction between 'Stress' and 'Stress-related illness' is a nuance that you really won't care about if you are its victim. This - like the messy intersection - is an aspect which is blown out of proportion due to the legalities of blame; it is not directly relevant or helpful in dealing with Stress itself.
OK; rant over. For those who just love definitions and semantics:
Stress is an adverse reaction to excessive pressure or unrealistic demands, arising when an individual worries that they can't cope. 'Pressure' is frequently motivating; 'Stress' is typically debilitating.
Each individual can therefore decide, based on their own mental state, whether they are suffering from stress. We use the acronym SFS - suffering from stress - to avoid any ambiguity or confusion regarding causation or diagnosis. In other words: if you say you're stressed, OK you're stressed. 'SFS' implies that, not only do you say that you're stressed, but that you are displaying the physical and mental symptoms of stress. We also throw in SRI - stress-related illness - to describe any medical conditions (depression, anxiety) which are caused by or linked to Stress, but we don't go into that, 'cos that is a clinical diagnosis. Does the average teacher need to know this? Almost certainly not.
Summary:
Are you stressed? Only you can say.
Do you have a stress-related illness? Only a doctor can say
Are you suffering from stress (SFS)? Depends on how many symptoms you are displaying....