The name is an acronym which stands for "Both Sides Up" or "Both Sides Utilized" a reference to the two ways a BOSU ball can be positioned. It is flat on one side and a raised dome on the other. The solid platform is 25 inches in diameter and the dome should be inflated until it is firm. Using a BOSU ball is another way of adding variety into your workouts in a functional way. It focuses on developing balance in your left and right sides and also efficient timing of the up and down movement.
When the dome side faces up, the BOSU ball provides an unstable surface while the device remains stable. In this position it is often used for balance training; however, you can walk, run, jump or hop on it which means you can use it for an aerobic workout too.
If you flip it over the BOSU is extremely unstable and there are a huge number of exercises you can do on it in this position too. This combination of stable/unstable means a wide range of people can use it, elderly people, injured people all the way up to elite level athletes.
The BOSU can also be used for core conditioning and stabilization and can provide great proprioception challenges to your training.