Well, right off the bat if it is illegal then use is considered abuse. If it is legal by prescription but not prescribed to you. If you take more of your medication than prescribed. if you use an OTC drug in a manner inconsistent with its labeling.

OK, those are all the well defined ones. But what it really comes down to is using a substance to avoid problems instead of fixing them or to escape from things rather than facing them. And then there is the simple answer - if you have to ask, you are probably abusing it.

No judgement; I have abused many, many substances in my time.