Which instrument measures temperature?

A thermometer, and the hard part historically was not building one but agreeing what it should say. Early instruments were open to the air and therefore measured pressure as much as heat; sealing the liquid in a tube, in the 1600s, is what made comparison between two instruments meaningful at all.

Then everyone chose different reference points. Fahrenheit used a freezing brine mixture for zero and something close to human body temperature near the top, which is why water freezes at the peculiar figure of 32. Celsius used water's freezing and boiling points and originally put boiling at zero, with the scale reversed after his death. Both are arbitrary, and neither has a real bottom.

Kelvin does. There is a genuine coldest possible temperature, absolute zero, at which the thermal motion of particles is at its minimum, and the kelvin scale starts there. It is unreachable - approaching it gets exponentially harder - though laboratories have got within billionths of a degree, and the coldest known place in the universe is not in space but inside an experiment on Earth.