what is the difference between undefined and not defined in JavaScript

what is the difference between undefined and not defined in JavaScript

Code:

//not defined

//undefined
var a,b;

Chapters :

0:00 - Introduction
0:21 - Writing the code and execution for not defined
0:55 - Writing the code and execution for undefined
2:22 - End Tags and subscription to channel

Description Tags:

Donation Section:
Now you can Donate us via Paypal or Google Pay

My second Channel:

Next Steps :

---------------------------------------------------------------------------------------------------------------------------------------------------------
---------------------------------------------------------------------------------------------------------------------------------------------------------
Must Watch Playlists

---------------------------------------------------------------------------------------------------------------------------------------------------------
---------------------------------------------------------------------------------------------------------------------------------------------------------

#undefined
#notdefined
#javascript