near-light speed; sub-light speed
Technical term used in physics and science fiction to describe speeds approaching but not reaching the speed of light.
Designing a spacecraft that travels at near-light speed is still a theoretical challenge.
Simulate the behavior of particles at sub-light speed.
亜光速 is a near-synonym also meaning 'sub-light speed'; 準光速 is slightly more common in modern technical contexts.
Compound of 準 (jun, 'quasi-', 'semi-') and 光速 (kōsoku, 'speed of light').