A monolith is a mysterious black slab discovered throughout the Solar System in Arthur C. Clarke’s Space Odyssey Universe. The monoliths were created by an unseen alien race, known only as the Firstborn.
The monoliths are capable of many different functions. One served as a catalyst for evolution, encouraging the development of sapience in lifeforms. Another was a sort of "burglar alarm," alerting the aliens that the humans had left Earth. The Jupiter monolith served as a gateway to a space transit system, and once created the "Star Child," an entity of pure energy who functioned as a probe for the Firstborn.
The monoliths could also replicate themselves, generating a cloud, or rather a giant disc, made up of millions of monoliths. One such assemblage was formed to increase the mass of Jupiter, inducing nuclear fusion and igniting the gas giant, turning it into a star.