In Python, a function is a block of code that can be executed multiple times from different parts of the program. It's a way to group a set of statements together and reuse them in different parts of the program.

Here's a step-by-step guide on how to define a function in Python:

**1. Define the function**

In Python, you define a function by using the `def` keyword followed by the function name and parentheses `()`.

For example:
```
def greet(name):
    print("Hello, " + name + "!")
```
In this example, `greet` is the function name, and `name` is the parameter.

**2. Define the function body**

The function body is the section of code that contains the actual logic of the function. It's indented with spaces or tabs.

In this example, the function body is:
```
print("Hello, " + name + "!")
```
**3. Define the function parameters**

The function parameters are the values that the function takes as input. They are indented with spaces or tabs.

In this example, the function parameters are:
```
name = "John"
```
**4. Define the function return value**

The function return value is the output of the function. It's indented with spaces or tabs