3D printing is any of various processes in which material is joined or solidified under computer control to create a three-dimensional object, with material being added together (such as liquid molecules or powder grains being fused together), typically layer by layer. In the '90s, 3D printing techniques were considered suitable only to the production of functional or aesthetical prototypes and, back then, a more comprehensive term for 3D printing was rapid prototyping. Today, the precision, repeatability and material range have increased to the point that 3D printing is considered as an industrial production technology, with the name of additive manufacturing. 3D printed objects can have a very complex shape or geometry and are always produced starting from a digital 3D model or a CAD file.
There are many different 3D printing processes, that can be grouped into seven categories:

Vat photopolymerization
Material jetting
Binder jetting
Powder bed fusion
Material extrusion
Directed energy deposition
Sheet laminationThe most common by number of users is a material extrusion technique called fused deposition modeling (FDM). This builds a three-dimensional object from a computer-aided design (CAD) model, usually by successively adding material layer by layer, unlike the conventional machining process, where material is removed from a stock item.The term "3D printing" originally referred to a process that deposits a binder material onto a powder bed with inkjet printer heads layer by layer. More recently, the term is being used in popular vernacular to encompass a wider variety of additive manufacturing techniques. United States and global technical standards use the official term additive manufacturing for this broader sense.