BJD stands for Ball-Jointed Doll. These dolls are typically made of resin, and have articulated joints held together by elastic cords that run inside the doll’s body and limbs. BJDs are highly customizable – you can change many parts of the doll including hair/wigs, eyes, hands, feet, clothing, and face-up (painting on the face to create unique expressions).
Mechanical Joint Dolls (MJDs) are a type of articulated doll that features joints with mechanical components, allowing for a wider range of movement and posing compared to traditional ball-jointed dolls (BJDs). While BJDs typically use elastic cords to hold the joints together, MJD joints are made of plastic or other mechanical material to achieve articulation. Like BJD, you can also change many parts of MJD to suit your tastes!
In the Doll Product Info tabs below, I have specified which are BJD and which are MJD.