Known Space is the setting of some dozen science fiction novels and several collections of short stories mainly written by author Larry Niven. It has also in part been used as a shared universe in the Man-Kzin Wars spin-off anthologies sub-series.
The epithet "Known Space" refers to a relatively small part of the galaxy centered around Earth. In the future which the series depicts, ranging from a few centuries to about a millennium from now, this region has been explored by humans and a number of its worlds have been colonized. On other worlds contact has been made with alien species such as the two-headed Pierson's Puppeteers and the aggressive felinoid Kzinti. The fictional universe is also the home of species outside Known Space as such, for instance the hominid inhabitants of a megastructure called Ringworld; the Ringworld orbits a sun outside of the spatial "region" known as Known Space, but it is a well-established artifact within the Known Space "universe". The stories span approximately one thousand years of future history, from the first human explorations of the Solar System to the colonization of dozens of nearby systems. Late in the series, this area is an irregularly shaped "bubble" about 60 light-years across.
The stories that comprise the Known Space series were originally conceived as two separate series, the Belter stories, featuring solar-system colonization and slower-than-light travel with fusion-powered and Bussard ramjet ships, and the Neutron Star/Ringworld series of stories, set much further into the future, which feature faster-than-light ships using "hyperdrive". The two timelines were implicitly joined by Niven in the story A Relic of the Empire, in which the background elements of the Slaver civilization (introduced in World of Ptavvs, from the Belter series) was used as a plot element of a story in the faster-than-light setting. Roughly 300 years separates the timeline of the last stories of the early setting (which are set roughly between 2000 and 2350), from the earliest stories in the later Neutron Star/Ringworld setting (which are set in 2651 (Neutron Star) and later). In the late 1980s, Niven opened up this gap in the known space timeline as a shared universe, and the stories of the Man-Kzin Wars volumes fill in that history, joining the two settings.