An immersive video is basically a video recording of a real world scene, where the view in every direction is recorded at the same time. During playback the viewer has control of the viewing direction, up down & sideways. Generally the only area that can't be viewed is the view toward the camera support. The material is recorded as data which when played back through a software player allows the user control of the viewing direction and playback speed. The player control is typically via a mouse or other sensing device and the playback view is typically 4:3 window on a computer display or projection screen or other presentation device such as a head mounted display.
A recent example of immersive video is provided by the Kogeto Dot panoramic video capture device and associated online community for smartphones (iPhone 4 and iPhone 4s as of December 3, 2011): see http://kogeto.com/dotspots/
Sometimes the phrase "immersive video" is used to describe full-size teleconferencing capabilities, such as Cisco's Telepresence product.