An ohmmeter is a device used to measure electrical resistance in ohms, which is a unit of measurement for resistance. It helps determine how well a material conducts electricity. Ohmmeters are commonly found in multimeters, which can measure voltage and current as well. When using an ohmmeter, the device sends a small current through the component being tested. The resistance is then calculated based on the voltage drop across the component. This tool is essential for diagnosing electrical issues in circuits and ensuring that components like resistors and wires are functioning properly.