1. (Informal) Something causing excitement or stimulating interest.
2. Something that pleases you and causes you to gain interest in something or someone is a turn-on. Something about a person of the opposite sex that causes you to become sexually or romantically interested in that person is a turn-on. A turnoff is the opposite of a turn-on.

1. To become hostile towards; SYN. turn upon.
2. To produce suddenly or automatically, as of tears of charm.
3. When people or animals that you had good relations with turn on you, they stop being friendly and try to hurt you.

When you turn on an electrical or mechanical device, you change the controls to make it start producing or doing something. Turn off is the opposite of turn on. Switch on is the same as turn on.

Something that turns you on pleases you and causes you to gain interest in something or someone. Something about a person of the opposite sex that turns you on causes you to become sexually or romantically interested in that erson. Turn off is the opposite of turn on.