The New Oxford American Dictionary defines hijack as a verb, meaning "to illegally seize (an aircraft, ship, or vehicle) in transit and force it to go to a different destination or use it for one's own purposes."
The two common elements in all classic definitions of hijacking are goods and transit.
Newer editions of popular dictionaries include diversion of purpose in a second definition of hijack, one not involving transit or solid goods. Someone can hijack a company, hijack a meeting or hijack a computer network by using it for a purpose other than that for which it was intended.
"Skyjack" and "carjack" are two common variations on the word "hijack," involving airplanes and automobiles, respectively; piracy is, technically, a form of hijacking.
The word first saw use by the American press in 1923, though the exact origin of the term is unknown. It may have something to do with the "highwaymen" who robbed coaches.