walled garden

Short definition from Wiktionary

Noun

1. 1
   A garden enclosed by walls for purposes of shelter or decoration.
2. 2
   A closed platform for content or services controlled by a single carrier or service provider, typically requiring subscription to access.
3. 3
   A set of web pages or other resources that are richly interlinked and share a common focus, but have little or no linkage to or from the larger network.