What is a BJD?
First, we should define what a Ball-Jointed Doll (BJD) is. Dolls seem to have existed for as long as humans have. The only limitations were what time was allotted for creating art, and what technology of materials had been developed. Most of the first jointed dolls were made of wood. At first, these has simple tongue and groove joints. As technology improved, we began to see ball joints. Compared to the tongue and groove joints, which could only move in the direction that the groove was cut, ball joints allowed for a much broader range of motion.
Dolls also began to be held together by tension. While our contemporary ABJD use elastic, rubber bands were not invented until the mid-1800s, and the elastic bungie like cords we used were not created until much after that. Instead, the tension was provided by springs placed inside a hollow body. Strings were tied to the springs, pulling them and creating tension.
These dolls were created from a mix or "composite" of materials that were formed in a mold. If you asked a doll collector in 1990 what a ball-jointed doll was, a ball-jointed composite doll would have been what came to their minds.
I'll let our podcast episode on the history of Volks and the Super Dollfie get into the specifics from the cross pollination of culture, but in 1999 a doll with a similar concept of construction to the composite BJD came to the consumer market. These dolls are made of resin, have ball joints, and are strung together with elastic. The Super Dollfie is recognized as the beginning of consumer available "Asian" Ball-Jointed Dolls.
The wigs of the dolls are typically not glued on, and are interchangeable. Joints are usually present on the neck, shoulders, elbows, wrists, hips, knees and ankles. However sometimes the dolls have more (ex. Domuya Flexi Body) and sometimes have less (ex. Volks Res Tenshi).
Concurrently to the development of the Super Dollfie, Volks also developed the Dollfie Dream. These have hollow parts just like the Super Dollfie, but were made from vinyl rather than resin. At first their parts were strung together, but eventually an ABS plastic internal skeleton/armature was developed for the dolls. While Dollfie Dream and other counterparts made by Smart Doll, Azone, and others technically lack a traditional ball joint, they are usually included in the ABJD discussion due to the similarities in scale, maintenance, and style. Some refer to this type of doll as "Internal Frame Dolls" or IFD in order to differentiate them from their elastic strung cousins. They are also referred to as "Anime Dolls", but I believe that term to be a bit too generic to apply now, as resin heads and 1/6 scale fashion dolls also have anime styled facial features.
Finally, the "Asian" part of ABJD refers to the fact that for the first 5-8 years of the hobby, the manufacturers and distributers of the dolls were in Japan, China, and Korea. As the hobby has become more popular in the "West", we have come to see more companies and artists like Kaye Wigs, Sweet Dolls, and others take their love of ABJD and create their own resin and elastic strung dolls for sale. Even these may be referred to as ABJD, due to their closer stylistic resemblance to the contemporary BJD than the composite, antique dolls.
Finally as Jeff Bridges said in Tron 2, "The game has changed." As 3D printing becomes more available for consumers and small businesses, the rate at which products are developed and sold is increasing. Some BJD artists use a 3D printed model to create a mold for casting, but more and more we are seeing print on demand or dolls being sold as 3D prints. They function just the same and as 3D printing technology advances, resolutions, colors, texture, and materials will improve to make even better printed BJD.
This was a rather long answer to a seemingly simple question, but ABJD are more complicated than a simple counting of joints and classification of materials used. They are a contemporary doll, and as such their meaning and place in culture are changing at the rate their collectors and manufacturers innovate. We won't have a true definition until the last ABJD is created, and hopefully that time will never come.