A visionary was someone who had a strong vision for the future.
In 2374, Section 31 placed Doctor Julian Bashir in an illusory scenario in which Weyoun 5 claimed him to secretly be a Dominion agent placed within Starfleet. When Bashir stated that he was saying he was a traitor, Weyoun told him that his friends on Deep Space 9 might vilify him, but that history would judge him to be both a great man and a visionary who helped bring an end to one of the most devastating wars the galaxy had seen. (DS9: "Inquisition")
In Vic Fontaine's holosuite program, Frankie Eyes once got to shake Bugsy Siegel's hand, noting how much of a disgrace it was that there was no statue of Bugsy in Las Vegas as Frankie regarded him to be a visionary. (DS9: "Badda-Bing, Badda-Bang")