means "true forging" and refers to a knife that is made from a single piece of hard steel - either white or blue and is forged by hand. It has a differential heat treatment to provide high edge retention with toughness. This is accomplished by putting mud on the upper parts of the blade and by leaving the edge bare so that it will react instantly to the quench. You can see the place where the mud stopped on the blade. It is called the hamon. We often call it a temper line in the West.