William Henry Fox Talbot's process to make negatives on paper; the image is formed from silver iodide coated on the base without a binder and is developed. The calotype process produced a translucent original negative image from which multiple positives could be made by simple contact printing. This gave it an important advantage over the daguerreotype process, which produced an opaque original positive that could only be duplicated by copying it with a camera. In the mid-20th century, the term was sometimes used incorrectly for any salted paper or paper negative process.

Source: AAT

http://vocab.getty.edu/page/aat/300053484