A dress form is a three-dimensional model of the torso used for fitting clothing that is being designed or sewed. When making a piece of clothing, it can be put on the dress form so one can see the fit and drape of the garment as it would appear on a body, and make adjustments or alterations. Dress forms come in all sizes and shapes for almost every article of clothing that can be made. Dress forms in standard clothing sizes are used to make patterns, while adjustable dress forms allow garments to be tailored to fit a specific individual.
This is often colloquially referred to as a Judy for the female form and a James for the male.
Media related to Dress forms at Wikimedia Commons