A maglock portal servo control
In Starfleet engineering, a maglock (short for magnetic lock) was an electromagnetic locking mechanism used to connect structural components together.
In April 2151, during a mission to a Suliban helix, Jonathan Archer used a device to reverse the polarity of any maglock within one hundred meters, causing the docked Suliban cell ships to scatter away from the helix. (ENT: "Broken Bow")
The eisillium deposits on Archer's Comet disrupted the maglock of the grapplers. (ENT: "Breaking the Ice")
Also in 2151, Starfleet officers aboard the NX-class starship Enterprise used a mag-lock to seal cabin E-14, Crewman Daniels' former quarters, and placed the device on the cabin's door. (ENT: "Cold Front")
To prepare a trap for a group of Ferengi pirates who were trying to ransack Enterprise and were insistent on searching for the ship's vault, Commander Tucker installed and activated a mag-lock on the door to an area aboard the vessel called Bio-Matter Resequencing. He later deactivated the device in order to allow the admission of three of the Ferengi, who were desperately seeking the vault as they believed gold was inside. However, they realized it was a trap and T'Pol, who was waiting inside, immediately stunned the Ferengi with a phase-pistol. (ENT: "Acquisition")
In the final draft script of "Acquisition", Tucker briefly uses the mag-lock to fool the Ferengi intruders into thinking the device is dangerous, Tucker making the mag-lock produce a series of small beeps before he disengages it.
In March 2152, Lieutenant Malcolm Reed deactivated the mag-lock on Daniels' quarters while Reed was with Captain Jonathan Archer, Enterprise's commanding officer. Reed, immediately thereafter, took the device off the door to Daniels' cabin. (ENT: "Shockwave") The mag-lock was subsequently used to lock the door again. However, Reed later unlocked it, took it off the door, and temporarily placed it on a bunk in the room while he retrieved a futuristic database from the cabin. Upon leaving Daniels' quarters, Reed returned the mag-lock to the door, re-sealing it. (ENT: "Shockwave, Part II") Later that year, the device was disengaged and removed from the door by Archer himself, this time accompanied by T'Pol. Immediately after entering the room, Archer placed the device on the bunk. He took it off the bed upon exiting. (ENT: "Future Tense")
The 22nd century style of mag-lock wasn't referred to as such in canon; it was called this in the final draft scripts of "Cold Front", "Acquisition", "Shockwave", "Shockwave, Part II" and "Future Tense".
In 2268, Montgomery Scott believed that pain bands which he, James T. Kirk, Leonard McCoy, and Spock were wearing used a magnetic lock. (TOS: "Spock's Brain")
In 2269, Spock believed that lock to the slumber chambers was controlled by a magnetized lock. (TAS: "The Lorelei Signal")
An example of the use of multiple maglocks was the deflector array installation of the Sovereign-class; the dish could be disconnected from the hull by unlocking a set of three maglocks. (Star Trek: First Contact)
The California-class used maglocks to secure the panels making up the outer hull to the ship. If these became unusuable, there where also manual access magclamps available to use from within the ship. (LD: "First First Contact")