An electric chair is a tool or instrument that's used to execute a convicted criminal by sending enough electricity through his body to kill him. The electric chair was invented as a humane means of execution.
The first electric chair was used in New York state in 1890, and the term electric chair was coined the previous year. At the time, the most common way to put a prisoner to death was by hanging, which was increasingly seen as cruel and inhumane. The electric chair was meant to cause death quickly and relatively painlessly, though in US states that still have the death penalty, it's more common today to execute criminals using lethal injection.