What is Doorway Page and what is the use of these pages in promotion of a website?

Doorway pages are web pages that are created to spam index and another name for it is aka gateway pages.

if a page is optimize targetting a single keyword is called as doorway page.google treat this page as a spam page and this is actually balck hat techniques.

Doorway pages are 1-2 pages website that contains different information for users & crawlers and redirect visitors to other web page. This is a black hat technique that is used to target more visitors through unethical method.

Doorway Pages are web pages designed and built specifically to draw search engine visitors to your website. They are standalone pages designed only to act as doorways to your site. The fact is that doorway pages don't do a very good job of generating traffic, even when they are done by "experts." Many users simply hit their back buttons when presented with a doorway page.Doorway pages also known as landing pages, are pages set up by a black hat SEO webmaster with the sole intention of attracting traffic via search engines, to his website.

Doorway pages are typically large sets of poor-quality pages where each page is optimized for a specific keyword or phrase. In many cases, doorway pages are written to rank for a particular phrase and then funnel users to a single destination. Whether deployed across many domains or established within one domain, doorway pages tend to frustrate users.