Albert Einstein was a famous physicist of 20th century Earth. He was born in Germany and was best known for his equation E = mc² and his works on the Theory of General Relativity. His influential work in what would become quantum theory laid the cornerstone for critical scientific breakthroughs in the centuries to come, including transporters and warp spaceflight.
In 2268, when discussing genius with Leonard McCoy, James T. Kirk said, "Genius doesn't work on an assembly line business. Did Einstein, Kazanga, or Sitar of Vulcan produce new and revolutionary theories on a regular basis?" (TOS: "The Ultimate Computer")
In 2285, David Marcus believed that if Project Genesis had worked, his mother, Carol Marcus, would be remembered in the same breath as Isaac Newton, Albert Einstein, and Surak. (Star Trek II: The Wrath of Khan)
In 2369, Lieutenant Commander Data created holodeck versions of Albert Einstein, Sir Isaac Newton and Stephen Hawking in order to watch how three of the greatest Human scientists would interact during a game of poker. Data's observations were "most illuminating". (TNG: "Descent")
In 2374, Captain Kathryn Janeway referred to Einstein in her Captain's log when she was in search of the omega molecule, and was forced to impose the Omega Directive. In her log, she noted that, in spite of her apprehensiveness, she now knew how Albert Einstein must have felt when he had developed the atom bomb, watching helplessly as science had taken a destructive course. Janeway, however, also noted that, unlike both Einstein and Carol Marcus, she had at least a chance to prevent science from becoming subverted. (VOY: "The Omega Directive")
When a hologram of Crell Moset proposed a medical procedure endangering a cytoplasmic lifeform, which had attached itself to B'Elanna Torres, in 2375 The Doctor protested as this lifeform could be its species' Einstein or Picasso. (VOY: "Nothing Human")