This is probably not what they say, but the unambiguous expansion of the term would be “Don’t put employee computers on internal networks and don’t rely on network topology for authentication.”

Probably something like "limited access" or "authenticated users". Could also include micro segmentation.

The basics are pretty straight forward, but every vendor has stretched to include their key capabilities which makes the idea murky.

BeyondCorp, NoVPN, EndToEndSec, something like that