An early method of execution involving being tied or nailed to a pole.
Unlike what the name seems to imply, crucifixion can still take place even if a cross is not present. Some examples (NSFW): post #1129852, post #1302606.
The crucifixion of Jesus of Nazareth and his propitiation on the cross is central to Christianity, believed by Christians to be a divine sacrifice for the atonement of sins. The crucifixion of Christ is widely alluded to in media, although, in some cases, the allusion is purely aesthetic.