In casting, a pattern is a replica of the object to be cast, used to prepare the cavity
into which molten material will be poured during the casting process. Patterns used in sand casting may be made of wood, metal, plastics or other materials. The making of patterns, called pattern-making, is a skilled trade that is related to the trades of tool and die making and mold-making, but also often incorporates elements of fine woodworking.
Those were the traditional definition of pattern making wherein you spend lots of time and money at the cost of limitations. 3D printed parts can be easily used as the pattern for mold making in metal casting, thus increasing the range of complex material that can be molded. Using 3D Printers, you can develop your cast in less than a day.