A domain used specifically to rank well in search engines for particular keywords, serving as an entry point through which visitors pass to the main domain.
Doorway domains are doorway pages taken to the next level. Rather than adding some pages to an existing site for search engine optimization purposes, a new site is set aside to be completely optimized.
Doorway domains are often easy to spot:
see also: SEO Tools
SEO Courses – compare at the big list of online courses.