Single-Input Single-Output (SISO) refers to a system where one input is processed to produce one output. This concept is commonly used in control systems and signal processing, where a single variable is manipulated to achieve a desired result. For example, a simple thermostat that adjusts the temperature based on one temperature sensor is a SISO system. In contrast to more complex systems, SISO systems are easier to analyze and design. They are often represented using mathematical models, such as transfer functions, which describe the relationship between the input and output. SISO systems are foundational in fields like engineering and automation.