Microsurgery was a form of surgery requiring an operating microscope.
In 2367, Beverly Crusher said that without the interactive signals of the Borg implants that connected Jean-Luc Picard to the Borg Collective, it would only be a matter of microsurgery to free him. (TNG: "The Best of Both Worlds, Part II")
In 2378, Tuvok listed performing microsurgery as one of The Doctor's skills that set him apart from being akin to a replicator, as Ardon Broht asserted. (VOY: "Author, Author")
In an alternate 2379, Doctor Kes performed microsurgery on Captain Chakotay's elbow a few weeks prior to the onset of the morilogium. (VOY: "Before and After")