noun
measuring instrument; measuring device; gauge
General term for instruments used to measure physical quantities such as length, weight, temperature, or electrical values.
この測定器で温度を測ります。
I measure the temperature with this measuring instrument.
工場ではさまざまな測定器が使われている。
Various measuring instruments are used in the factory.
計器 refers specifically to meters or gauges, often with a display, while 測定器 is a broader term for any measuring device.
Compound of 測定 (measurement) + 器 (instrument, device).