Complex sentence: In the first half of the 1980s, he coined the term "Kōseiki Sekai" (光世紀世界, Light Century Universe), defined as the sphere with a diamerer of 100 light years (within 50 light years radius from the Sun).
Split into simpler sentences: In the first half of the 1980s, he made a term "Kōseiki Sekai" (Light Century Universe). That's as defined the sphere with a diamerer of 100 light years (within 50 light years radius from the Sun).