Stiffness (especially muscle stiffness) is a condition that makes a part of the body (usually a joint) hard to move. It becomes difficult for the bone in that joint to move freely. As a result, the person cannot move as easily as before.
The word "stiff" comes from Old English and Germanic languages. It means rigid, inflexible, or hard to bend.
Stiffness is caused by diseases such as Swine influenza. But is also causes by sitting for too long, exercise, Aging, Dehydration, and others.