A tool or technique used to identify the nature or cause of a condition, situation, or problem. The mechanic used a diagnostic to determine why the car wouldn't start.
A message or report that provides information about the status or condition of a system or process. The software displayed a diagnostic indicating that the update was successful.
(usually plural) The procedures, tests, or data used in identifying the nature or cause of a malfunction, disease, or problem. The technician ran diagnostics on the server to locate the fault.