The third dimension refers to a world opposite to the "second dimension", that is, the "dimension" we exist, that is, the real world we live in.
The word "dimension" originated from Japanese acgn works, which usually refers to the fantasy world and the collection of its various elements.
A rule and order is completely different from the reader's existing world. For example, the world in which magic or steel bullets exist is often called "different dimensional world", or simply "different dimensional world".
Three dimensional dolls and other objects should be three-dimensional roles from the original definition, but they are usually based on the three-dimensional of two-dimensional roles, or emphasize their virtual essence, so they are sometimes called "2.5-dimensional roles".
Based on the above etymology, those who only have interest or feelings for the virtual characters in ACG, but are not interested in the real opposite sex, are jokingly called "residents of the second dimensional world", and those in serious cases are jokingly called the second dimensional abstinence syndrome.