‘射线’ (shè xiàn) literally combines ‘射’ (to shoot, emit) and ‘线’ (line, ray), forming a scientific term meaning 'ray' — especially in physics and medicine. It refers to streams of energy or particles traveling in straight lines, such as X-rays, gamma rays, or cosmic rays. Unlike the English word 'ray' used in geometry (e.g., light ray), 射线 in Chinese almost always carries a technical, often medical or physical, connotation.

The term appears frequently in healthcare contexts (e.g., X-ray imaging, radiation therapy) and scientific education. It is rarely used metaphorically or in casual speech. While ‘光线’ (guāngxiàn) means ‘light ray’ in everyday contexts, 射线 specifically denotes ionizing or high-energy radiation. Learners should avoid substituting it for general terms like ‘beam’ or ‘line’ unless referring to scientifically defined radiation.