Adherence refers to a patient's ability and willingness to follow recommended health practices.
Likewise it is the degree to which a patient exactly follows a prescribed treatment regimen
Other /More definition:
Adherence refers to the degree to which an individual follows a recommended health-related or illness-related recommendation; a patient's ability and willingness to follow recommended health practices.
It is also the extent to which a patient’s behavior matches with his or her practitioner’s advice.