Hybrid problems are those that involve both continuous and discrete parameters or processes. At is simplest this may be a dataset some continuous valued features and discrete valued features. It may also refer to underlying systems that include both physical dynamics and digital control. In the formal methods community the term 'hybrid systems' refers to the latter. On the whole, designing algorithms to deal with both continuous and discrete aspects is harder than either on their own.
Used on Chap. 4: pages 75, 76