In Arthur C. Clarke's Space Odyssey, Monoliths are machines built by an unseen extraterrestrial species. In the series of novels (and the films based on these), three Monoliths are discovered in the solar system by humans. The response of the characters to their discovery drives the plot of the series. It also influences the fictional history of the series, particularly by encouraging humankind to progress with technological development and space travel.
The first Monolith appears at the beginning of the story, set in prehistoric times. It is discovered by a group of hominids and somehow triggers a considerable shift in evolution, starting with the ability to use tools and weaponry. It is later revealed that thousands of other monoliths exist elsewhere in the solar system.