A minefield was a large number of mines deployed in space. Several races know how to make use of them.
The Romulan Star Empire was known to deploy minefields in the 22nd century to protect the sovereignty of their borders. The Earth Starfleet starship Enterprise strayed into a cloaked Romulan minefield in April 2152. One mine breached its hull while another attached itself to the ship. (ENT: "Minefield")
After the Klingon Empire withdrew from the Khitomer Accords and the Federation-Klingon War began, the Klingons attempted to mine the Bajoran system, thus preventing the Dominion from leaving the system if they invaded. This would have left the Empire able to focus on conquering the entire Cardassian Union and the rest of the Alpha and Beta Quadrants. A cloaked mine detonated and was detected by a runabout from Deep Space 9. Later, another mine detonated and a Template:ShipClass ship, the IKS Drovana, was badly damaged. This led to the failure of the Klingons' plan as Worf and Kurn boarded the ship and obtained the detonation codes. The USS Defiant detonated the minefield and the other Klingon ships retreated back to the Empire. (DS9: "Sons of Mogh")
As war with the Dominion seemed increasingly likely in late 2373, the Federation mined the entrance to the Bajoran wormhole, to stop more Dominion reinforcements coming through. The mines were self-replicating and were equipped with cloaking devices. Even after the Dominion seized control of Deep Space 9, the minefield remained in place. The field stood for at least three months before it was destroyed after disabling the replication units with an antigraviton beam. (DS9: "Call to Arms", "A Time to Stand", "Sacrifice of Angels")
The planet Salina Prime in the Delta Quadrant was protected by subspace minefields. In 2375, a Druoda series 5 long-range tactical armor unit took control of the USS Voyager and plotted a course around the minefields in an attempt to the reach the planet. (VOY: "Warhead")