An islet in the estuary of the Thames, at the mouth of the Medway, belonging to Kent, from which it is separated by the Swale (spanned by a swing-bridge); great clay cliffs rise on the N., and like the rest of the island, are rich in interesting fossil remains; corn is grown, and large flocks of sheep raised; chief town is Sheerness, where the bulk of the people are gathered; is gradually diminishing before the encroaching sea.