4D film or 4-D film is a marketing term for an entertainment presentation system combining a 3D film with physical effects that occur in the theatre in synchronization with the film. Effects simulated in a 4D film may include rain, wind, strobe lights, and vibration. Seats in 4D venues may vibrate or move a few centimeters during the presentations. Other common chair effects include air jets, water sprays, and leg and back ticklers. Hall effects may include smoke, rain, lightning, air bubbles, and smell.
Because physical effects can be expensive to install, 4D films are most often presented in custom-built theatres at special venues such as theme parks and amusement parks. However, some movie theatres have the ability to present 4D versions of wide-release 3D films. The films Journey to the Center of the Earth (2008) and Avatar (2009) are among the films that have received a 4D treatment in certain theatres.Also there are mobile 4D theaters, which are mounted inside vehicles such as enclosed trailers, buses and trucks.
There is no consistent standard for using the term "4D" when describing the presentation of a film. 4D films have also occasionally been marketed as 5D, 6D, 7D, 8D, 9D, 11D, 15D and so on in order to emphasize the variety or uniqueness of their theatre effects. This is scientifically different from actual fourth dimensional space. Notable formats for providing different aspects of a "fourth dimension" to films include Human 4D, Sensurround, Smell-O-Vision and 4DX. As of June 2015, about 530 screens worldwide have installed the technology.