Bi-Level refers to a type of design or structure that features two distinct levels or layers. This concept is commonly seen in architecture, where buildings may have two floors, or in transportation, such as bi-level trains that have two decks for passengers. The design allows for more efficient use of space and can enhance functionality. In the context of mathematics or optimization, bi-level programming involves two levels of decision-making, where one problem is nested within another. The upper level typically represents a leader's decision, while the lower level represents a follower's response. This approach is useful in various fields, including economics and operations research.