The `def` keyword in Elixir is used to define functions. It indicates the start of a function definition, allowing developers to encapsulate behavior and logic.

In Elixir, the def keyword is a fundamental construct used to define functions, marking the beginning of a function definition. Functions are the primary building blocks of Elixir code, encapsulating behavior and logic that can be reused throughout your application. When you define a function using def, you specify the function's name and any parameters it takes, followed by the function's implementation. For example, you might define a simple function like this: def hello(name), do: "Hello, #{name}!". The def keyword helps promote clear and modular code by allowing developers to organize their logic into reusable functions. Functions can also have multiple clauses, enabling pattern matching based on different input shapes. This feature enhances the expressiveness of your code, as you can handle various cases with ease. Additionally, the def keyword can be used in modules, where you can define public functions that can be accessed from outside the module. Overall, the def keyword is essential for structuring Elixir applications and defining the core behaviors of your code.