The term 'Birthday Slice' refers to a piece of CAKE that is traditionally served on someone's birthday. The crossword solution for this term is 'CAKE, ' which is a four-letter word. CAKE is a type of sweet dessert that is often made with flour, sugar, eggs, and butter. It is usually decorated with icing or frosting and can be flavored with various ingredients such as chocolate, vanilla, or fruit. Birthday cakes are often decorated with candles and other festive decorations.