Definition of "Bust" from Wikipedia:
A bust is a sculpted or cast representation of the upper part of the human figure, depicting a person's head and neck, as well as a variable portion of the chest and shoulders. The piece is normally supported by a plinth. These forms recreate the likeness of an individual. These may be of any suitable material (such as marble, bronze or clay).
The term Mini-Bust came into the statue and collectibles industry to describe the small series of mostly-superhero-themed busts measuring less than 10 inches or so. The term was popularized by Bowen Designs during their early Marvel Comics sculpts and has since become the common nomenclature.