Most history books list the years from 27 B.C. to 180 A.D., which is from the time that Augustus Octavian took power until the time that Marcus Aurelius died, as the "Pax Romana" - "the Roman peace".
In reality, this was an era filled with nearly constant military action. The empire was defending itself against attempted foreign invasions, and also against domestic uprisings in various provinces and annexed territories. Marcus Aurelius himself spent most of his career, not in Rome, but fighting in Europe with the army.
Perhaps the reason that this violent and war-torn era was named "the Roman peace" was because it was a time when, in terms of domestic politics, the authority and the power of the emperor remained relatively unchallenged. For nearly a century, prior to 27 B.C., the Roman world had been savaged by a vicious civil war. Only in comparison with this bloody power struggle of Roman against Roman could the later era be seen as peaceful.
If not a time of "peace," it could perhaps at least be described as an era of domestic political stability.