One medical appointment I described PEM, not using the phrase (they wouldn't have recognised it) but described the experience, in as simple a way as I could given time constraints & the sense the professional was disinterested anyway. I focused on the sickness aspect: that activity makes me ill in a characteristic way and loss of function. I very very purposely avoided the words "fatigue" or "tired" or any related synonym. To me they are very clearly a separate concept and experience. I wanted the professional to listen and recognise the specific illness response / function loss of PEM.