What is a doorway page?
There are various definitions for the term "doorway page".
Some SEO professionals consider that it is a page created specifically for a purpose related to SEO whose content would be ultra-optimized.
These pages would mostly be entry pages, with very little emphasis on the rest of the website.
However, I will add a crucial element to this definition to qualify or not a doorway page: la redirection.
It is actually possible to redirect the internauts directly to another more "marketing" page so they cannot see the ultra-optimized content intended for robots.
Aware of these methods, search engines are today capable of noticing very easily this method and penalize most of the time very severely these pages.
As a consequence, we have to be very careful and so tend to no longer use these methods easy to set up (and easy to unmask too).