A "First-Class Function" is a concept in programming where functions are treated as first-class citizens. This means that functions can be assigned to variables, passed as arguments to other functions, and returned from other functions, just like any other data type. This flexibility allows for more dynamic and modular code. In languages that support first-class functions, such as JavaScript and Python, developers can create higher-order functions. These are functions that can take other functions as input or produce them as output, enabling powerful programming techniques like callbacks and functional programming patterns.