Harrier jets are an aircraft capable of vertical take off and landing. Macbeth owns two harrier jets, and Banquo and Fleance used them to land on the H.M.S. Churchill to steal the Scrolls of Merlin. David Xanatos also owns harrier jets, but they were in the shop at the time of the theft. ("A Lighthouse in the Sea of Time")
Real World Background
The Harrier Jump Jet, formally referred to as the "Harrier" or informally as "the Jump Jet", is a British-designed military jet aircraft capable of Vertical/Short Takeoff and Landing (V/STOL) via thrust vectoring. The Harrier family is the only truly successful design of this type from the many that arose in the 1960s.
There are four main versions of the Harrier family: Hawker Siddeley Harrier, British Aerospace Sea Harrier, Boeing/BAE Systems AV-8B Harrier II, and BAE Systems/Boeing Harrier II. The Hawker Siddeley Harrier is the first generation version and is also known as the AV-8A Harrier. The Sea Harrier is a naval strike/air defence fighter. The AV-8B and BAE Harrier II are the US and British variants respectively of the second generation Harrier aircraft.