A device designed to visualize small things. Usually refers to normal light microscopes, but can refer to more esoteric things like electron micrscopes.
The earliest notable use of the microscope was by the 18th century Dutch scientist Anton (or Antonie) van Leeuwenhoek (1632-1723). He discovered, using a microscope, such things as bacteria, free-living and parasitic microscopic protists, sperm cells, blood cells, microscopic nematodes, rotifers, and other fun stuff. What makes things more amazing is that his microscopes were not much more than magnifying glasses.
These days, there are many variations on the light microscope, especially when used in the biomedical sciences:
Of course with today's modern computerized image gathering techniques, there are various real-time image gathering setups.
Notable modern manufacturers of research-grade light microscopes include Nikon, Canon, Zeiss, Leica, and Olympus. Interestingly, many of these same companies make cameras, and had their beginning in the late 19th and early 20th century when optics were being worked out.