Guzheng is the romanization of 古箏. 古 (gǔ) means old or ancient. 筝 (zhēng) refers to a zither-type instrument, something with strings stretched between two bridges. The two together, “guzheng”, means a zither with fixed bridges on both ends and moveable bridges in the center.
Most modern guzheng have 21 or more strings but 16 and 18-string guzheng are still made. 100 years ago instruments with 16 metal strings were the most popular. Nylon-coated metal strings were invented in the 1950s and are the current favorite.