This page contains information regarding new Star Trek material, and thus may contain spoilers.
A micro-wormhole was a wormhole of very small size. 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. (VOY: "Eye of the Needle")
The Red Angel generated a micro-wormhole to travel through space and time. Because wormholes were inherently unstable, the Angel had to hold the wormhole open with a membrane that was anchored to her original time. During an attempt to trap the Angel in 2257, NCIA-93 used a graviton beam to seal the wormhole behind her. (DIS: "The Red Angel")
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")