noun
near light speed; sublight speed
Technical term used in physics and science fiction to describe speeds close to, but below, the speed of light.
亜光速で移動できる宇宙船の設計が研究されている。
Designs for spacecraft capable of traveling at near light speed are being researched.
SF小説では、亜光速航行がよく登場する。
Sublight travel often appears in science fiction novels.
超光速 means faster-than-light, the opposite of 亜光速.
Compound of 亜 (a, 'sub-') + 光速 (kōsoku, 'speed of light'). The prefix 亜 indicates something just below or approaching the following concept.