This somewhat amorphous body of study is known under such names and guises as:
Operations Research (a US-ism)
Operational Research (a UK-ism)
OR
Management Science
Decision Sciences
Systems Science
Mathematical Modelling
Mathematical Programming
A not unreasonable definition is provided by CORS :
Operational Research (O.R.) helps to improve operations in business and governments through the use of scientific methods and the development of specialized techniques. O.R. provides systematic and general approaches to problem solving and decision making, regardless of the nature of the system, product, or service. The approaches and tools used in O.R. are based on one or more of mathematical methods, simulation, and qualitative or logical reasoning. Many of these tools and approaches depend on computer based methodologies for implementation.
O.R. originated in Great Britain during World War II to bring mathematical or quantitative approaches to bear on military operations. Since then O.R. has evolved to be applicable to the management of all aspects of a system, product, or service, and hence is often referred to as Systems Science or Management Science. It has now become recognized as an important input to decision-making in a wide variety of applications in business, industry, and government.