Stereolithography (SL) is one of several methods used to create 3D-printed objects. It's the process by which a uniquely designed 3D printing machine, called a stereolithograph apparatus (SLA) converts liquid plastic into solid objects.
The process was patented as a means of rapid prototyping in 1986 by Charles Hull, co-founder of 3D Systems, Inc., a leader in the 3D printing industry.