The "dimension" referred to in Japanese ACG works usually refers to the fantasy world in the works and the aggregation of its various elements. For example, a rule and order is completely different from the reader's existing world, such as the world where magic or steel bullets exist, which is often called "otherdimensional world" or simply "otherdimensional world".
In addition, traditionally, virtual characters represented by plane media, such as characters in cartoons or animations, are often called "secondary characters" because of their two-dimensional nature, so as to distinguish them from real (three-dimensional) characters. However, the characters made of three-dimensional computer images are called "2.5-dimensional characters" because they are in the virtual world and have three-dimensional concepts.
Three-dimensional dolls and other things should be three-dimensional roles according to the original definition; However, it is sometimes called "2.5-dimensional role" because it is usually based on the three-dimensional role of the second dimension, or emphasizes its virtual nature.
Based on the above etymology, those who are only interested in the virtual role appearing in ACG and have no interest in the real opposite sex are dubbed as "people living in the second dimension", and in serious cases, they are dubbed as "the second dimension forbidden syndrome".