It looks to me like a Metal Oxide Varistor or MOV for short.
These are used as surge supressors.
Like any surge supressor they are sacrificial in that they supress a voltage surge by allowing current to pass through them.
The thing that is confusing in the photo is that the breaker its on appears to have four connections. Generally in shore poower we have two breakers side by side. The handles are generally connected and the cases are generally connected but they are built as two separate breakers each having two wire landing points.
In this case you have what appear to be four wire landing points on this breaker unless I'm being fooled by the photo. I'm guessing here as to their use but possiible an auxillary contact for a indicator light or something like that. Its hard to tell from the photo. I've never seen a breaker with 4 lugs on it before.