Portal

A portal was a means of travelling across time, space, and/or universes. The transmat technology of several species took the form of a portal. (COMIC: The Dalek Project, Supremacy of the Cybermen)

Notable examples
The Space-Time Manipulator generated a portal which allowed travel across spacetime. (TV: The Cambridge Spy)

When the Dalek Restoration Empire invaded Islos, the Archivians evacuated the population via a portal in the Archive of Islos. They also used the portal to summon an entity that had agreed to safeguard the populace in exchange for the Emperor of the Restoration. (WC: The Archive of Islos) Using beam projection technology, the Daleks were able to open portals back to the Entity's dimension, banishing both the Entity and the Mechanoids that they had duped into aiding them. (WC: The Deadly Ally, Day of Reckoning)

During the Last Great Time War, the Time Lords used blue portals. The Doctor had experience with this technology, and believed that such portals no longer existed following the Time War. (COMIC: Supremacy of the Cybermen)

In the war's early stages, the Valeyard managed to wipe out the Dalek Empire, save the Dalek Time Strategist who used a transdimensional portal to escape into the multiverse. (AUDIO: The War Valeyard) The Strategist emerged from a similar portal in on the parallel Skaro of another reality built by the native Davros. Wiring itself to this portal, the Time Strategist was able to use the energies of the multiverse to restore the Daleks to N-Space. (AUDIO: Restoration of the Daleks)

When a rogue Type 1 TARDIS found itself in the Void, it generated white portals of Void energy to consume N-Space, viewing the universe as a chaotic place, before the Doctor's TARDIS convinced it to halt this scheme, the portals being used to, largely, return everything to its proper place and time. (COMIC: The Lost Dimension)

After the vortex manipulator of Leon Perkins was sabotaged, his hand appeared out of a portal to the Tenth, Eleventh and Twelfth Doctors. When the portal appeared before the Thirteenth Doctor, she managed to pull Perkins out. (COMIC: A New Beginning, The Road to...)

The passage between N-Space and the Solitract plane took the form of a portal. The Thirteenth Doctor encountered one of these portals in Norway in 2018. (TV: It Takes You Away)

In the far future, the Boundary was a portal on a distant planet that human refugees used to flee the remnants of the Cyber-Empire. managed to connect this portal to the ruins of Gallifrey, allowing direct passage between the two planets. When he showed the Doctor the Matrix record of the Timeless Child, he claimed that the Child had arrived from its own realm through a similar looking portal. (TV: Ascension of the Cybermen, The Timeless Children)

In an alternate timeline, following the coup against Rassilon, Rassilon allied himself with a colony of Cybermen at the end of the universe, granting them the means to conquer all of history. By the 24th century of this timeline, the Cybus Cybermen used the blue portals formerly used by the Time Lords in the Last Great Time War to quickly transport entire armies of Cybermen onto planets and commence instantaneous invasions; these portals were efficient to the point that the Cybermen ceased using Cyber-Fleets for invasions.

An example of these portals' use was the Cybermen's invasion of Sontar, where an army of Cybus Cybermen descended upon the planet's surface and immediately attacked the Last Great Sontaran War Fleet. The Tenth Doctor was stunned to see the Cybermen using these Gallifreyan portals, leaving himself exposed to a Cyberman emerging from a portal opening behind him; however, he was saved by the sacrifice of the Sontaran-Prime. The Cyberiad ultimately lost this technology when, on Gallifrey at the end of the universe, a betrayed Rassilon and the Twelfth Doctor used the Eye of Harmony to regenerate the universe and undo the Cybermen's conquests across time. (COMIC: Supremacy of the Cybermen)