Known Space is the fictional setting of several science fiction novels and short stories 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" is an in-universe term that 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 aliens have been encountered, 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.
Source: Wikipedia, "Known World," available under the CC-BY-SA License.