A ball-jointed doll is any doll that is articulated with ball and socket joints. In contemporary usage when referring to modern dolls, and particularly when using the acronyms BJD or ABJD it usually refers to modern Asian ball-jointed dolls.
These are cast in polyurethane resin, a hard, dense plastic, and the parts strung together with a thick elastic. They are predominantly manufactured in Japan and South Korea, and in 2006 Chinese manufacturers entered the market. BJD design is both realistic and influenced by anime with proportionally large heads and big eyes.
They range in size from about 60 cm (2 feet) for the larger dolls, 40 cm (16 in) for the mini dolls, and all the way down to 10 cm (4 in) or so for the tiniest of the tiny BJDs. BJDs are made to be easy to customize, by painting, changing the eyes and wig, and so forth.
The modern BJD market began with Volks line of Super Dollfie in 1999. Super Dollfie, or just Dollfie, are sometimes erroneously used as generic blanket terms to refer to all Asian BJDs regardless of manufacturer.
But Super Dollfie is a registered trademark for Volks line of BJD, and Dollfie is the trademarked name of their line of Barbie sized 1/6 scale vinyl dolls, which are not proper ball-jointed dolls at all.