A microscope is a mechanical instrument that magnifies the image of small objects. You would use a microscope to look at something tiny, like an amoeba.
First used in the 1650s, microscope is descended from the Modern Latin microscopium, meaning "an instrument for viewing what is small." In science, microscopes are essential for examining material that can't be seen with the naked eye, like bacteria and viruses. In the same vein, you might feel like you're "under a microscope" if you're being very closely observed and scrutinized, like a star in the limelight or a politician in front of the media.