Same as gateway page.
A doorway page is a webpage created to rank for specific (most often long-tail) keywords while sending visitors to a different page. It is one of the elements of black-hat SEO.
For Google, doorway pages are web spam because they have no value for the searchers. Besides, doorway pages can send visitors to malware or phishing websites. That is why Google is constantly improving its anti-spam algorithms that help them detect and punish doorway pages.