Final answer:
The primary reason for a patient's outpatient visit is identified by the principal diagnosis, which is determined after study to be the main cause of the healthcare visit.
Step-by-step explanation:
The diagnosis that gives the primary reason for a patient's outpatient visit is known as the principal diagnosis. This is the condition established after study to be chiefly responsible for occasioning the patient's visit to the healthcare facility. In contrast, a primary diagnosis may be used more broadly in inpatient settings, a secondary diagnosis refers to a condition that coexists with the primary condition or develops during the hospital stay, a probable diagnosis is used when the condition is suspected but not confirmed, and a Z code diagnosis is primarily used to describe reasons for healthcare encounters other than a disease or injury.