A Function Block Diagram (FBD) is a graphical representation used in automation and control systems to depict the functional relationships between input and output signals, devices, or processes. It visually organizes blocks that represent different functions, such as logic gates, timers, or control systems. Each block has inputs and outputs, and the blocks are interconnected to show the flow of data and control signals within a system.

FBDs are commonly used in programming of Programmable Logic Controllers (PLCs) and are part of the IEC 61131-3 standard for industrial control programming languages. The diagram allows for intuitive programming and visualization of processes by showing how individual functions interact to form a larger control system.