A micro-wormhole was a wormhole of much smaller size than their counterparts. A naturally-occurring micro-wormhole was originally of a much greater size, but was in an advanced state of decay, and over a great stretch of time had degraded to a smaller size.
In 2371, the USS Voyager discovered a naturally-occurring micro-wormhole thirty centimeters in diameter, which Captain Janeway speculated had been collapsing for centuries. The wormhole led from the Delta Quadrant to Romulan space (in the past). The crew gave the wormhole the name Harry Kim wormhole. (VOY: "Eye of the Needle")
In 2376, the Pathfinder Project at Starfleet Communications was able to create a temporary, artificial micro-wormhole using the MIDAS array, to briefly communicate with Voyager over 30,000 light years away in the Delta Quadrant. (VOY: "Pathfinder")