Damascus steel is a steel used in Middle Eastern swordmaking from about 1100 to 1700 AD. Damascus swords were of legendary sharpness and strength, and were apocryphally claimed to be able to cut through more "ordinary" European swords and even rock. The exact technique used to create original Damascus steel is up for debate.
Damascus swords often had an obvious patterned texture on their surface. Several other steelmaking techniques also result in patterned surfaces, and have often been sold as Damascus steel, Damascened steel and sometimes watered steel. The most common technique for producing these materials is the pattern welding, which is today widely used for custom knife making. Skilled swordsmiths can manipulate the patterns to mimic the complex designs found in the surface of the original, medieval Damascus steel.
Another theory behind the hardness of Damascus steel is that the steel contains a small amount of vanadium, which would theoretically strengthen the blade.