Question

What is an interferometer?

Answer 1

A device to measure differences between electromagnetic wavelengths.

Explanation:

Interferometry is a family of techniques in which waves, usually electromagnetic waves, are superimposed causing the phenomenon of interference in order to extract information.

Diagrams and discussion here:
https://www.ligo.caltech.edu/page/what-is-interferometer

and here:
https://science.howstuffworks.com/dictionary/astronomy-terms/interferometer-info.htm