Diagnostic means designed to identify a problem or determine what's wrong with something. When you take a diagnostic test in school, it isn't graded. Instead, it helps your teacher figure out what you already know and what you still need to learn. A mechanic runs diagnostic tests on a car's computer to pinpoint why the engine light is on.

The word comes from medicine, where doctors use diagnostic tools to figure out what's causing a patient's symptoms. An X-ray is a diagnostic tool that lets doctors see broken bones. Blood tests provide diagnostic information about how your body is functioning.

In any field, something diagnostic reveals the nature or cause of a problem so you can address it properly. A computer technician might run diagnostic software to find out why your laptop keeps freezing. A scientist might perform diagnostic experiments to understand why a plant isn't growing. The key idea is that diagnostic tools and tests don't fix problems, they identify them. Once you know what's wrong, you can figure out the right solution. Without accurate diagnosis, you might waste time treating the wrong problem entirely.