"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, where a single variable is controlled to achieve a desired result. For example, a thermostat that adjusts the temperature based on a single temperature reading is a SISO system. In contrast to more complex systems, SISO systems are simpler to analyze and design. They are often used in various applications, including engineering, electronics, and signal processing. Understanding SISO systems is fundamental for developing more intricate systems that may involve multiple inputs and outputs.