Adherence refers to taking HIV medication consistently and correctly.
That means taking medication at the right times, in the right amounts, in correct combinations, before or after meals; precisely as prescribed by the doctor. This is essential in the treatment of HIV/AIDS.
When people don't adhere to their treatment regimens, they risk viral mutations causing new viral strains to develop in their bodies that are resistant to medication. This can make effective treatment difficult or even impossible.

A different term sometimes used for adherence is compliance.