To set a flow of fluid or gas running by rotating a tap or valve.
Turn on the tap
To power up, to put into operation, to start, to activate (an appliance, light, mechanism, functionality etc.).
Please turn on automatic updates.
(of a device) To start operating; to power up, to become on.
My computer won't turn on.
To fill with enthusiasm; to intoxicate, give pleasure to ( + to an object of interest or excitement).
Attractive packaging can turn buyers on to a product.
To sexually arouse.
To cause to take up drugs, especially hallucinogens.
To violently rebel against; to suddenly attack (this is the intransitive verb turn, with on functioning as a preposition not as an adverbial particle).
Suddenly all his friends turned on him.
To depend upon; to pivot around, to have as a central subject (this is the intransitive verb turn, with on functioning as a preposition not as an adverbial particle).
The argument turned on the question of whether or not jobs would be lost.