An interferometer is a device built to measure the interference between two different waves.
The most popular type of interferometer is the Michelson interferometer, which was built by Albert Michelson in the 1880s in an attempt to experimentally detect the luminous ether through the Michelson-Morley experiment (along with the chemist Edward Morley).
The design of the Michelson interferometer split a beam of light in opposite directions, with the idea that if the Earth was moving through the luminous ether, then the beam moving in the same direction as the ether would move faster than the beam moving against the ether, just like swimming with the current will move you faster than swimming against the current. The beam of light is then reflected back, so that all paths travel the same distance to arrive at the same screen. If the speeds were different, the device would demonstrate interference patterns.
This experiment was ultimately unsuccessful at detecting the ether, but this failure had nothing to do with a flaw in the experiment. It turns out that the luminous ether doesn't exist! In fact, the experiment is still conducted in modern physics courses on college campuses across the world, where students continue to verify that the ether doesn't appear to exist ... a discovery which paved the way for our modern understanding of wave-particle duality.