Fenced frames
An embedded frame deliberately stripped of all communication with the page around it. Built so it can display something chosen from data neither party is allowed to see.
An ordinary iframe leaks in both directions: the embedding page knows the address it set, and the two can talk through messages. That is incompatible with the idea that an ad should be selected from a user's interests without the advertiser learning who she is or the site learning what was chosen. A fenced frame solves it by cutting the channels. It is loaded not from an address but from an opaque configuration object produced by an auction inside the browser, message passing upward does not exist, and the frame's access to storage and network is constrained so it cannot smuggle out identifiers on its own. That makes the construct technically suited to Chrome's ad auctions, and the line between what is genuinely partitioned and what merely looks partitioned is precisely the detail the criticism has aimed at.