Definition of tusche in US English:
 tusche
noun ˈto͝oSH(ə)ˈto͝oSH(ə)
A greasy black composition, in liquid form or to be mixed with liquids, used as ink for making lithographic drawings.
 Example sentencesExamples
-  Draw onto the plates with lithographic pencils, crayons, tusche, and process with a single solution.
 -  The diluted tusche dries to resemble ` toad-skin’ washes usually found in zinc-plate lithography.
 -  The design is drawn on a special, flat stone or on a metal plate with a greasy water-repellant substance (greased crayon or a greasy ink called tusche).
 -  The grease in the lithographer's crayon or tusche will repel water during the lithographic process.
 -  The artist draws an image on acetate or Mylar, ‘with washes, tusches, lithographic crayons or whatever… that's where the magic is’ he said.
 
Origin
  
Early 20th century: from German tuschen, from French toucher 'to touch'.