In sailing, what does 'dead calm' mean?

One of the most important aspects of sailing is wind, and those who love the sport are often looking for the perfect gusts to carry them to their destination. But what happens when there is no wind? Dead calm is a common term used by sailors to describe the complete absence of wind, causing the water to become flat and motionless. This phrase dates back to the 16th century, when sailors believed that a calm sea signaled an ominous future. They believed that it might be an indication that death was nearby, hence the term ‘dead calm.’

Source: Myblue4you