Now, to be honest, we're kind of sloppy sometimes at TM4T. We don't think it's particularly helpful to draw nice distinctions between 'stress' and 'burnout' or make nuanced points about stress not really being an illness. However, in order to understand burnout, we have to be really specific about what stress is.
Stress relates to how you feel at any particular moment. So: it is subjective – if you feel that you are stressed, then you are stressed. It can also be short-term and intermittent: if you say you are stressed one minute, and not stressed the next, that's fine. No one can contradict you. If a gym instructor yells at you and makes you do push-ups right to your limit, then you might feel stressed, but the solution is clear: stop doing push-ups and tell the gym instructor to piss off.
Of course, usually in a school, stress doesn't work like that. You can't just make-it-stop and you are more likely to experience stress several days in a row, especially at busy times, or as deadlines loom. However, as term moves on and deadlines pass, stress often lessens or disappears entirely. It is often, therefore, a self-mending thing.
Burnout is different: it isn't subjective, short-term, intermittent or self-mending, or easy-to-control.
Burnout often takes place over an extended period of time. You might gradually come to believe that your work is meaningless; gradually recognise a disconnect between what you're doing now and what you originally wanted to do. You end up going through the motions instead of being fully engaged. Eventually you display cynicism, exhaustion, and frequently poor teaching performance. To the outside observer, you might not even look particularly stressed, just disinterested and incompetent.