The Leonardo da Vinci holoprogram was created aboard the U.S.S. Voyager in the 24th century, based on the 15th/16th century artist and scientist. He would become an adviser to Kathryn Janeway and a subject of study for The Doctor.
As Leonardo was never known to have used a surname in the modern sense, "Leonardo da Vinci" or "Leonardo" are the correct names to use for him. However, he is frequently, in Star Trek: Voyager and elsewhere, referred to as "da Vinci". This is inaccurate because "da Vinci" is not a surname at all; it means "of Vinci" or "from Vinci," Leonardo's home town, and actually refers to where Leonardo was from.