Call to Action (Call-To-Action Trigger, CTAC) (In terms of interactive TV) -

1) The opportunity for the viewer to interact with specific television programming.  A hot spot such as an icon, link or some sort of other notification symbol, may appear on your display and the viewer then could  interact with it through the TV system by using a remote control device such as an air mouse, standard remote or device incorporating 3D gesture recognition technology.

2) During the interactive programming, a specific time may come when viewers are offered via interactive TV a product or service if they contact the sponsor or content provider. This could be a free item such as a product sample, or it could be a coupon, etc. These products or services could be delivered by email and also to other corresponding devices such as cell phones or PCs. 

3) That point in time when the viewer is informed by the TV program (show), perhaps by an announcer, that the time for interactivity had arrived.