to take control of a vehicle illegally and by force, especially a plane, ship, bus, or car etc that is moving
The plane was hijacked by unknown terrorists.
to take control of a computer, phone, webcam, website, or a URL etc illegally
Hackers have created a website to hijack the webcam feeds of more than 33,000 people.
to take control of an event or a situation without the organizers or people officially involved wanting your involvement, especially illegally, by force, or in an otherwise unwanted way
Environmental activists hijacked Pepsi's recent launch of its "green" soda.