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, ensuring they function correctly. Ohmmeters can be standalone devices or integrated into multimeters, which measure voltage and current as well. When using an ohmmeter, the user connects the probes to the component being tested, and the device displays the resistance value, helping to diagnose issues in electrical circuits or devices.