CORSMiddleware is not defined

`CORSMiddleware` is not a built-in module or function in Python. It is likely that you are trying to use this middleware in a web framework like Flask or Django, and you need to import it from the appropriate package.

For example, in Flask, you can import `CORS` from the `flask_cors` package and use it like this:

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(app)

In Django, you can import `CorsMiddleware` from the `corsheaders` package and add it to the `MIDDLEWARE` setting in your `settings.py` file:

MIDDLEWARE = [
    # ...
    'corsheaders.middleware.CorsMiddleware',
    # ...
]

Make sure you have installed the required packages before using them. You can do this by running `pip install <package-name>` in your command prompt or terminal.