What is "photochrom"

Photochrom, sometimes spelled Fotochrom or Photochrome (although this term more often refers to later photographic postcards), and also called the Aäc process, is a process for producing colorized images from black-and-white photographic negatives via the direct photographic transfer of a negative onto lithographic printing plates. The process is a photographic variant of chromolithography, a broader term that refers to color lithography in general.