Ohmmeter
Definition
An ohmmeter measures electrical resistance directly — dial it to Ω, touch the probes to a component, and read its resistance.
Technical definition
An ohmmeter applies a small known current and measures the resulting voltage, computing R = V/I. It must measure components with the power off, since any external voltage corrupts the reading.
Example
Testing a supposedly 220 Ω resistor that reads 217 Ω is fine (within tolerance); one that reads 'OL' (overload) is burnt open.
Common confusion
Never use an ohmmeter on a live circuit. The meter injects its own current to measure resistance — external voltage fights it, giving nonsense readings and possibly damaging the meter.