When Vladimir Putin first mentioned Novorossiya over a year ago, alarm bells went off in the Ukrainian capital of Kiev.

The czarist-era term, which means New Russia, refers to a large swath of modern-day Ukraine that Catherine the Great won from the Ottomans and Cossacks at the end of the 18th century. Hearing it from the Russian president in April 2014 reinforced fears that Moscow's designs on its neighbor extended beyond the Crimean peninsula and across Ukraine's southern shores to Odessa.

But now, as abruptly as the Novorossiya movement appeared, it has begun to fade. Mr. Putin has stopped using the term. More modest borders for the Russia-backed rebel republics in eastern Ukraine have begun settling into place. The word is heard less and less on Russian state television.