The term " plum cake " and " fruit cake " have become interchangeable . Since dried fruit is used as a sweetening agent and any dried fruit used to be described as " plums " , many plum cakes and plum puddings do not contain the plum fruit now known by that name . ( Plum pudding is a similar , richer dish prepared with similar ingredients , cooked by steaming the mixture rather than baking it . ) The term " plum " originally referred to prunes , raisins or grapes . Thus the so @-@ called plums from which English plum puddings are made " was always raisins , not the plump juicy fruits that the name suggest today . "