An ohmmeter is a device used to measure electrical resistance in ohms, which is a unit of measurement for how much a material opposes the flow of electric current. It is commonly used in electronics and electrical engineering to test components like resistors, wires, and circuits to ensure they are functioning properly. Ohmmeters can be standalone devices or integrated into multimeters, which measure multiple electrical properties. When using an ohmmeter, the user connects the probes to the component being tested, and the device displays the resistance value. This helps in diagnosing issues in electrical systems and ensuring safety in various applications.