A balanced incomplete block design (BIBD) is a statistical design used in experiments where each treatment is not applied to every experimental unit, allowing for more efficient use of resources. In a BIBD, each treatment appears in a fixed number of blocks, and every pair of treatments is represented together in the same block a specific number of times. This design balances the trade-off between the number of treatments and the number of blocks to reduce variability and enhance the accuracy of estimates.