Paraplegia is the medical name for losing the ability to move the legs and the lower part of the body. It is usually caused by damage to the spinal cord — the thick bundle of nerves running down inside your backbone — from an accident, an illness, or sometimes something a person is born with. When that cord is cut off or crushed, the messages from the brain cannot reach below the injury, so the legs no longer move. Feeling is often lost too, and control of the bladder and bowels is frequently affected as well, which is a part of the condition that people outside medicine tend not to think about. The word names the condition itself, not the person: the person is a paraplegic. It is uncountable, so you say 'he has paraplegia', never 'a paraplegia' or 'paraplegias'. It sits in a small family of similar medical words worth knowing together: paraplegia is the lower body; quadriplegia (also called tetraplegia) is all four limbs; hemiplegia is one side of the body, often after a stroke. Note that paraplegia does not mean the person cannot do things — many people with paraplegia work, drive adapted cars, and compete in sport. The word describes movement, not ability.

下半身癱瘓、截癱（醫學上稱「下肢癱瘓」）。台灣日常也常說「半身不遂」，不過那個說法比較鬆，中風造成的單側癱瘓也會被這樣叫，講精確一點還是「下半身癱瘓」。