At a minimal level, it is a revolution that has been able to withstand and defeat the counter-revolution (inevitably heavily supported by international capital, as with the ‘civil’ war in Russia). When it has done so, it can gain some precious space to begin the process of constructing socialism.
That was the answer I gave then. But I suggest there is another part to the answer: a successful revolution provides inspiration for other revolutionary movements. Let me give one example, from the 1930s in China and the sheer inspirational power of the Russian Revolution among Chinese communists.
America, England, France, Germany, Japan, Italy, and other capitalist or imperialist powers had sent thousands of political, cultural, economic, or missionary workers into China, actively to propagandize the Chinese masses with credos of their own states. Yet for many years the Russians had not had a single school, church, or even debating society in China where Marxist-Leninist doctrines could legally be preached. Their influence, except in the soviet districts, had been largely indirect. Moreover, it had been aggressively opposed everywhere by the Kuomintang. Yet few who had been in China during that decade, and conscious of the society in which they lived, would dispute the contention that Marxism, the Russian Revolution, and the new society of the Soviet Union had probably made more profound impressions on the Chinese people than all Christian missionary influences combined (Edgar Snow, RSOC, 352-53).