A doorway domain is a keyword rich domain or an exact match domain (EMD) created specifically to obtain good search engine ranking for the matching queries (queries with keyword or expressions encompassed in the doorway domain name).
The only purpose of the doorway domain is to capture search engine traffic related to the targeted keywords and to redirect it to the main website. To prevent detection by search engines, automatic redirection must not be used.
For some websites dozens of doorway domains may be used.
The golden days of doorway domains are gone. Google and other search engines use signals detection to identify the practice and the weight given to the presence of keywords in domain name for ranking is lower.
Nevertheless, doorway domains may still have some effect in sectors where SEO competition is less intense as in some B2B niche sectors.
See also doorway page.