To define a user defined function in Python, you can use the def keyword followed by the function name and a set of parentheses that may include arguments.

1) Here is an example of a simple function definition that takes no arguments and prints a message:

def greet():
   print("Hello, world!")

2) To define a function that takes arguments, you can include the names of the arguments in the parentheses. For example:

def greet(name):
   print("Hello, " + name + "!")

The above function takes a single argument name and prints a personalized greeting.

3) To define a function that returns a value, you can use the return keyword followed by the value you want to return. For example:

def square(x):
   return x*x

The above function takes a single argument x and returns the result of x*x.

I hope these examples help to illustrate how to define a user defined functions in Python.