An adaptive response of cells and organisms to a moderate (usually intermittent) stress.
What does that mean?
A good simple example of hormesis is exercise. When you exercise you’re putting your body under a moderate amount of stress - you’re actually doing a small amount of (temporary) damage to your body. If you hit the gym to lift some weights or go out for a long walk or run you are creating micro trauma - such as tiny tears in your muscles. When you get some rest later, interesting things start to happen - your body adapts to the stress of the exercise and it actually overcompensates by making you stronger and fitter. This is hormesis in action.
Another good example is diet. It’s fascinating to note that one of the reasons all these so called “superfoods” are good for us is actually because they contain substances that are mildly toxic! They activate pathways in the body that strengthen your immune system and reduce inflammation. Hormesis again.
Other examples you might think of are vaccinations, sunlight, intermittent fasting, saunas, cold showers and so on. These things are all beneficial in the right moderate dose.
A Swiss chemist called Paracelsus was probably onto hormesis a long time ago. Back in the 16th century he wrote:
All things are poison and nothing is without poison, only the dose permits something not to be poisonous.