According to the Oxford Dictionary hypnosis is "the induction of a state of consciousness in which a person apparently loses the power of voluntary action and is highly responsive to suggestion or direction".
According to the Etymology Dictionary, the word hypnosis comes from the name of the Greek god of sleep, hypnos. "He resided in Erebos, the land of eternal darkness, beyond the gates of the rising sun. From there he rose into the sky each night in the train of his mother Nyx (Night). Hypnos was often paired with his twin brother Thanatos (Peaceful Death), and the Oneiroi (Dreams) were his brothers or sons" in words of Theoi Encyclopedia.
Other words referring to hypnos are hypnophobia, hypnotherapy or hypnotic. We can see the use of the word hypnosis in this article.