The Leonardo da Vinci hologram was a hologram and created as part of a holographic program in the 24th century. (VOY: "Scorpion", "The Raven", "Scientific Method", "Concerning Flight", "The Omega Directive")
Kathryn Janeway of the USS Voyager enjoyed visiting the holographic Leonardo on the holodeck. He became an adviser to her, along with providing a haven during times of stress. One such example was when Captain Janeway was unsure of how to approach an alliance with the Borg. Leonardo counseled the captain that everything is a matter of interpretation and perspective and that she could turn the situation to her advantage – it was just a matter of being creative in devising a solution. Leonardo and Janeway unsuccessfully attempted to fly in a glider they constructed, which sent them crashing into a river. This led to the holographic Leonardo having a crisis of purpose and leaving many of his works unfinished. 'Catarina', as he called her, often referred to Leonardo by the honorific 'Maestro', and frequently employed occasional words of Italian in her conversations with him.
Once when Voyager was attacked by pirates who used a high-energy transporter beam to steal items of technological value, Leonardo's program was also stolen. He was used by the leader of the pirates, Tau, to develop new weapons which Tau sold to the highest bidder. Janeway and Tuvok tracked him and the rest of the stolen goods to a province controlled by Tau. He believed that he was in America and told Janeway that his patron was prince of this land. Leonardo was enjoying his new adventure because of the new inventions he was creating, but agreed to return to "Europe" when reminded by Janeway that his presence there was important and that she needed his help. Tau captured Janeway and drew a weapon on her. Leonardo came to her defense and knocked out Tau, then he and Janeway escaped successfully flying his glider through the air until Voyager got in range and beamed them and the glider into Cargo Bay 1 before jumping to warp. (VOY: "Scorpion", "The Raven", "Scientific Method", "Concerning Flight", "The Omega Directive")