In Christianity, Docetism is the belief that Jesus Christ did not have a physical body: his body was an illusion, as was his crucifixion. This belief is most common attributed to the Gnostics, who believed that matter was evil, and hence that God could not take on a material body. This sort of statement, however, does not do justice to the so called docetism of the gnostics. It could be further explained as this - the human body is temporary, and the spirit is eternal - therefore, to say that the body of Jesus was an illusion and that his crucifixion as well, could be compared to the same way a Buddhist speaks about illusion; illusion is everything that is temporary, not everything that is not real.
Docetism was rejected by mainstream Christianity and largely died out during the first millennium A.D.