"TELLUS STABILITA"

A literal translation of the phrase could be "global stability" or "regional stability" and, as such, it could refer to Hadrian's policy of consolidating the existing borders of the Empire, in contrast to the Trajanic policy of further conquest. 

A more specific meaning is also intended here, however. "Tellus" is a goddess of the Earth, a Roman equivalent of the Greek Gaia, and "Tellus Stabilita" is thus the genius of the "Pacified Earth," sometimes depicted in Roman imagery.

Image of the Goddess Tellus, from the Ara Pacis (Altar to Peace) in Rome.
GNU Free Documentation LicenseImage of the Goddess Tellus, from the Ara Pacis (Altar to Peace) in Rome. - Credit: Chris Nas