A Macintosh is a type of primitive personal computer in use on Earth during the 20th and 21st centuries. Manufactured by Apple Computers, the Macintosh was first introduced in 1984, and was one of the first popular computers to use a graphical user interface.
Montgomery Scott encountered such a computer when he traveled back in time to 1986 from the late 23rd century. Tasked with enclosing a bay so that the crew of the late USS Enterprise could transport humpback whales to the 23rd century, Scott, Dr. McCoy, and Hikaru Sulu visited a company called Plexicorp. After a tour of the plant, Scott offered to show plant manager Dr. Marcus Walter Nichols a way of enclosing a water tank with a sheet only one inch thick, rather than six-inch sheets commonly in use at the time.
Nichols gave Scott access to his computer - which was a Macintosh Plus - to enter the formula for transparent aluminum. Scott at first didn't realize that the computer was from a time before computers had voice interfaces. When he was handed the mouse, he mistook it for a microphone, at which point Nichols told him to use the computer. Despite considering the keyboard to be quaint, Scott was able to use the computer to enter a formula for transparent aluminum. (TOS movie: Star Trek IV: The Voyage Home)