The fruit is citron, from: a thorny evergreen shrub or small tree (Citrus medica) native to India and widely cultivated for its large lemonlike fruits that have a thick warty rind; the fruit of this plant is candied and used in confections and fruitcakes.
My mother used to make fruitcakes that were heartily eaten by everyone. They were very moist, spicy, and fruity, and there was nothing better with a hot cup of coffee.
Hank