SISO stands for Single Input, Single Output, a term commonly used in control systems and signal processing. It describes a system that has one input and one output, making it simpler to analyze and design compared to more complex systems with multiple inputs or outputs. In a SISO system, the relationship between the input and output can be represented using mathematical models, such as transfer functions. This allows engineers to predict how the system will respond to various inputs, which is essential for applications in fields like automation, telecommunications, and electronics.