What keyword is used to define a function in Python?

In Python, the keyword used to define a function is "def". This keyword signals the beginning of a function definition and is followed by the function name, parentheses, and a colon. The use of "def" is essential as it helps distinguish function definitions from other types of statements in Python.

When you write a function, you typically start with "def", then specify the function name, which must adhere to the naming rules of Python. Following that, parentheses can include any parameters the function accepts, and after the colon, you include the indented block of code that makes up the function's body.

Using "def" is one of the fundamental aspects of creating reusable code blocks in Python that can be invoked whenever needed, promoting better organization and modularity in programming. The absence of proper keywords such as "def" would lead to syntax errors, as Python would not understand that a function is being defined.