Any technique used in medical diagnosis.
"The doctor ordered a new diagnostic scan to pinpoint the source of my persistent headaches."
In plain English: A diagnostic is a test used to figure out what is wrong with something.
"The doctor relied on her diagnostic skills to identify the rare illness quickly."
Usage: Use "diagnostic" as a noun to refer specifically to a test or procedure that identifies a disease or condition, such as a blood diagnostic. Avoid using it as a noun when you simply mean the general act of finding a problem, which should remain an adjective modifying another word like "test."
characteristic or indicative of a disease
"a diagnostic sign of yellow fever"
"a rash symptomatic of scarlet fever"
"symptomatic of insanity"
"a rise in crime symptomatic of social breakdown"
Of, or relating to diagnosis.
"The doctor ordered a diagnostic test to determine if the patient had contracted the virus."
In plain English: Something diagnostic is used to find out what is wrong with a problem or a person's health.
"The doctor gave us a diagnostic test to find out exactly what was wrong with my stomach pain."
Usage: Use "diagnostic" to describe something that helps identify or confirm a specific condition, such as a symptom that points clearly to an illness. Avoid using it simply to mean "helpful," as that usage is imprecise and incorrect in standard English.
The word diagnostic comes from the Ancient Greek term diagnōstikós, which originally meant "able to distinguish." It entered English with this core sense of identifying or recognizing specific conditions.