Defining words can change with illness severity - fatigue can be anything from tired onto mental and physical paralysis, what we mean by a word will often need explanation as the meaning changes.
Resting - sitting down chatting or laying down in a darkened room with sensory deprivation, not moving or thinking at just the right temperature (neither warm or cold)
Slowly - longer time than when fit and well or weeks months if at all achievable.
Over Exerted - gone outside the energy envelope but it will be paid back in a few days or relapse in all symptoms that does not recover to previous levels.
Activity - gentle exercise Tai Chi for example or been sat up so your can be cleaned as the days only activity.
There are many more words that will have different meanings for us as individuals, defining what we mean removes some of the confusion that can occur when friends/family, doctors, social workers, benefits agencies communicate with us.