New term: Trampolines
Update:
Another term widely used is feeder sites. That’s probably a better term.
—————
Had a new commenter here who’s commenting on black and white SEO. He had a new (for me) term for freebie sites and dyndns subdomains:
Rathamahata, I should probably tell you that there Google DOES ban subdomains and folders as well. Not just whole sites. It’s just that the volume of those trampolines make it difficult to catch all of them. This has been going on for a long time. But your term is good, so I’ll appropriate it.