The term 'doughnut centers' refers to the holes in the middle of doughnuts. These holes are usually created by using a doughnut cutter, which is a tool that cuts out the center of the doughnut. The doughnut centers can also be filled with various ingredients such as jam, custard, or cream. Doughnut centers are an essential part of the doughnut-making process and are often enjoyed as a snack or dessert.