Stiffness is the quality of being rigid or hard to bend. A brand-new pair of jeans often has stiffness: the denim fabric feels thick and unyielding until you wash and wear them a few times. A diving board needs some stiffness to spring you into the air, while a rope has almost no stiffness at all.

The word also describes how your body feels when muscles are tight and movement is difficult. After sitting still through a long movie or sleeping in an awkward position, you might wake up with stiffness in your neck or back. Athletes experience muscle stiffness after intense workouts. This kind of stiffness usually goes away once you start moving around and your muscles warm up.

Stiffness can describe behavior too. Someone who acts with stiffness seems formal, uncomfortable, or unable to relax. Picture a student giving a presentation who stands rigidly at the front of the class, speaking in a rehearsed monotone instead of speaking naturally. Or imagine meeting someone new who shakes your hand with stiff, mechanical movements instead of genuine warmth.

In engineering and physics, stiffness measures how much force is needed to bend or compress something. A steel beam has much more stiffness than a wooden one of the same size. Understanding stiffness helps engineers design everything from bridges to airplane wings.