Selected answers from the Dumb SEO Questions G+ community.
Alan Bleiweiss: An orphan page is isolated from the rest of the site - no way to get to it from the site itself, and typically, little or no way to get back to the site from that orphaned page.
Doorway pages are pages that exist entirely to mass-scale an artificial way to get organic visibility for many variations of phrases where the content really ultimately, from a human understanding perspective, is really all going to lead to a single destination page.
Page 1: Round Red Widgets
Page 2: Red Round Widgets
Page 3: Widgets - Red and Round
Page 4: Round Red Widgets For Sale
In such scenarios, there really should only be one page to begin with.
For Google`s definition of doorway pages see https://support.google.com/webmasters/answer/2721311?hl=en