A telescope placed in space, typically for the purpose of avoiding atmospheric distortion, background light, and attenuation of light by the atmosphere.
Performing astronomy from Earth's surface is limited by the filtering and distortion of electromagnetic radiation (scintillation or twinkling) due to the atmosphere. Some terrestrial telescopes (such as the Very Large Telescope) can reduce atmospheric effects with adaptive optics. A telescope orbiting Earth outside the atmosphere is subject neither to twinkling nor to light pollution from artificial light sources on Earth.
Space-based astronomy is even more important for frequency ranges which are outside the optical window and the radio window, the only two wavelength ranges of the electromagnetic spectrum that are not severely attenuated by the atmosphere. For example, X-ray astronomy is nearly impossible when done from Earth, and has reached its current importance in astronomy only due to orbiting X-ray telescopes such as the Chandra observatory and the XMM-Newton observatory. Infrared and ultraviolet are also greatly blocked.