A Luxury yacht (also super yacht, large yacht and mega-yacht) is a very expensive, privately owned, professionally crewed sailing or motor yacht.
There is no industry-wide standard for the difference between super-yacht and mega-yacht. The only legal distinction is between yachts above 24 metres (79 ft) and below, as in some countries those above 24 meters must have a permanent crew onboard.
This term began to appear at the beginning of the 20th century, when wealthy individuals constructed large private yachts for personal pleasure, such as Charles Henry Fletcher’s Jemima F. III, at 111 feet, the largest motor- powered yacht in the world in 1908. More examples of early luxury motor yachts include the Cox & King yachts, Charles L. Seabury and Company’s yachts, M/Y (motor yacht) Christina O, and M/Y Savarona. Early luxury sailing yachts include America’s Cup classic J class racers such as Endeavour and Sir Thomas Lipton’s Shamrock.
Source : wikipedia