A deadlock seal was a form of lock or a device added to a lock that was almost impregnable. It was more secure than most locks. The Doctor's sonic screwdriver could not open regular deadlock seals, (TV: Bad Wolf) but these locks could be keyed to specific sonic devices, as Matron Cofelia did with her sonic pen. (TV: Partners in Crime)
Features
Deadlocking made locks more difficult to open, at least with sonic devices. More sonic devices could be used in concert to break deadlock seals. (TV: Partners in Crime) Additional deadlocking could be applied, from double (TV: Voyage of the Damned) to triple. (TV: Partners in Crime, GAME: TARDIS)
It was possible to create deadlock seals with a sonic device. The Doctor "locked" several things during his travels, particularly the controls of his TARDIS, which even the Master could not unlock. (TV: 42, Utopia, The Sound of Drums, Last of the Time Lords, Invasion of the Bane)
Deadlocking did not make glass unbreakable. (TV: The Poison Sky) It did, however, make metal incapable of being melted, or at least heated, and could prevent cords and doors from being separated or opened physically. (TV: The Time of Angels)
Uses
The Big Brother House on the Game Station was fitted with deadlock seals after the contestants of an earlier edition of the show walked out. (TV: Bad Wolf)
The Krillitanes used deadlock seals on several objects in Deffry Vale High School. (TV: School Reunion)
The Saxon Master deadlocked the TARDIS from inside to prevent the Tenth Doctor entering to stop his escape. (TV: Utopia)
The Heavenly Hosts on board the Titanic were constructed with double deadlocked circuitry. (TV: Voyage of the Damned)
Midshipman Alonso Frame used a "maximum deadlock" to seal the bridge of the Titanic. (TV: Voyage of the Damned)
Adipose Industries Headquarters could be deadlocked, including the doors and windows. The deadlocks there could be activated remotely on command. The Inducer built into the building was said to be triple deadlocked. However, the Doctor was able to open these locks after acquiring the sonic pen of the owner of the building, Miss Foster. (TV: Partners in Crime)
ATMOS devices developed by Luke Rattigan for the Sontarans were deadlocked to their respective cars. They could deadlock the entire car. (TV: The Sontaran Stragem/The Poison Sky)
John Hart's hypersteel handcuffs had a deadlock seal. He used them to lock himself to Gwen Cooper. (TV: Kiss Kiss, Bang Bang)
Torchwood Three's Torchwood SUV was triple-deadlock sealed. (TV: Children of Earth: Day One)
One particular Weeping Angel created deadlock seals on structures that weren't equipped with any such device. (TV: The Time of Angels)
GSO Arctic Drilling Station had deadlocks in its security locks. (GAME: Blood of the Cybermen)
The vase containing the Entity had a "triple-deadlock forcefield". (GAME: TARDIS)
A time ship had a deadlock seal on its activation console. (TV: The Lodger)
The Flydon Maxima Research Base used a series of deadlock seals for protection. (PROSE: The Depths of Despair)
The Pandorica used deadlocks as a security bypass. (TV: The Pandorica Opens)
The lock for the cashier's office in a 21st century theatre had a deadlock seal. The Eleventh Doctor found this improbable, considering the technology level of Earth and such an unnecessary need for security. (PROSE: Snowfall)
The headquarters for Lyttle Monsters Inc had deadlock seals for their doors. (PROSE: Snowfall)
Cass, refusing to join the Eighth Doctor in his TARDIS because he was a Time Lord, closed and deadlocked a door on her gunship, preventing the Doctor from reaching her. (TV: The Night of the Doctor)
River Song asked Flemming to deadlock the baggage hold of the Harmony and Redemption containing the Doctor's TARDIS and the body of her husband Hydroflax. According to her, only a genius could escape from it. (TV: The Husbands of River Song)
The vault-like laboratory door at the SERVEYOUinc-operated United System Research Base was protected by a tenth-generation quantum-liquid-deadlock seal encryption. In addition, most sensitive files were deadlock-encrypted. (COMIC: The Sound of Our Voices)