The term UNDERLYING STRUCTURE can be summarized as the foundation or framework of something. It is often used to refer to the basic elements that make up a system, such as the infrastructure of a building or the components of a computer program. The term can also be used to describe the fundamental principles that govern a particular field of study or activity. In the crossword puzzle, the solution for the term UNDERLYING STRUCTURE is FABRIC, which refers to the material used to create a structure or form.