A blueprint is a reproduction of a technical drawing using a contact print process on light-sensitive sheets. Introduced by Sir John Herschel in 1842, the process allowed rapid and accurate production of an unlimited number of copies. It was widely used for over a century for the reproduction of specification drawings used in construction and industry. The blueprint process was characterized by white lines on a blue background, a negative of the original. The process was not able to reproduce color or shades of grey.
The process is now obsolete. It was first largely displaced by the diazo whiteprint process, and later by large-format xerographic photocopiers.
The term blueprint continues to be used less formally to refer to any floor plan (and even less formally, any type of plan). Practicing engineers, architects, and drafters just call them "drawings" or "prints".