Tri-Di or 3-V (short for tri-dimensional or three-dimensional visual) was a form of entertainment technology popular from the mid 21st century and into the 23rd century. It involved the recording and projection of a subject as a holographic image that could be viewed from multiple angles.
The genres and formats of two-dimensional entertainment -- including short form episodic programs of the television era feature length versions of film-era movies -- were applied to Tri-Di shows. Enthusiasts of classic entertainment often attempted to trade copies of 3-V show tapes with others, and some ambitious technical specialists would often attempt to convert old two-dimensional visual recordings into the three-dimensional standard. (TOS novel: My Enemy, My Ally)
3-V took on great popularity in the 23rd century, so much so that other forms of entertainment, such as stage theater, began to be eclipsed in the eyes of those who still performed in old-style plays and stage shows. (TOS novelization: The Conscience of the King)