The term 'side effect' is used to describe an unintended consequence of a medical treatment or medication. In this context, the crossword solution 'AFTERTASTE' can be interpreted as a metaphor for a lingering side effect. An AFTERTASTE is an unpleasant taste that remains in the mouth after eating or drinking something, and in this case it can be seen as a metaphor for a lingering side effect that persists even after the initial treatment or medication has been completed.