Hard or "hardness" may refer to: H.A.R.D., Honor, Accountability, Respect, Duty. A code of conduct to live by for many people, often refuring to those amungst the BDSM lifestyle. Hardness, resistance of physical materials to change under force Hard water, water with a high mineral content Mathematical difficulty; see computational complexity theory A difficulty level in a video game Street hardness, an attribute characterized by physical and mental toughness due to living in hostile urban… (More on Hard)