Sectumsempra is a curse that was invented by Professor Severus Snape during his childhood when he was known as "The Half-Blood Prince". He created it with the intention of using it against his enemies, and it soon became one of his specialties. A rather dangerous curse, when the incantation is uttered its effect is the equivalent of an invisible sword; it is used to slash the victim from a distance.
Wounds inflicted by this spell can be cured by the incantation Vulnera Sanentur; the first time, the flow of blood eases, the second causes the wounds to knit and the third removes the worst effects of the curse. The victim would still require medical treatment, and if dittany is applied immediately, scarring can still be avoided.
Though the healing spell Vulnera Sanentur may heal the wounds and dittany may prevent scars, any body parts that have been severed by this curse cannot be grown back.