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. (Note that 4D films are not actually four-dimensional in the geometric sense of the word.) 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.
Effects simulated in a 4D film may include rain, wind, strobe lights, and vibration. Seats in 4D venues may vibrate or move a few inches during the presentation. Other common chair effects include air jets, water sprays, and leg and back ticklers. Hall effects may include smoke, rain, lightning, air bubbles, and special smells (for example, fireworks smells at the London Eye's Experience, and gassy smells when a stinkbug sprays it in It's Tough to Be a Bug).
4D films have occasionally been marketed as 5D, 6D, or 7D films in order to emphasize the variety or uniqueness of their theatre effects. However, there is no consistent standard among films for the application of these marketing labels.