Dimension

A dimension was a property of space, extending in a given direction, which, when combined with other dimensions of width and height and time, made up the Universe.

Dimensions within the universe
N-Space had eleven dimensions. (NA: Parasite) Space and time were two of these. They combined into "spacetime."

The First Doctor stated that the "fourth dimension" (as humans understood it) was time. (DW: An Unearthly Child) Vicki Pallister later elaborated that time, even though a dimension in itself, had dimensions of its own. The Doctor even stated, in their adventure at the time, that he did not understand "this" fourth dimension. (DW: The Space Museum)

Hyperspace was another dimension. (DW: The Stones of Blood) It could be reached using a black hole, which essentially collapsed dimensions into a singularity. (DW: The Horns of Nimon)

A TARDIS' interior existed as a separate dimension from the rest of the universe. This meant the interior didn't exist, creating temporal grace. (DW: The Hand of Fear) This may also explain why travel in a TARDIS took "no time at all," as the Third Doctor described. (DW: The Time Monster)

A location of space called the Arc of Infinity was known as an ancient curve between dimensions. (DW: Arc of Infinity)

N-Forms were constructs created by the Patrexes that were able to cross dimensions and exist in pocket dimensions of their own. (NA: Damaged Goods)

Other dimensions
According to Fenric, the Doctor trapped him in the Shadow Dimensions for seventeen centuries (DW: The Curse of Fenric) which were remnants of the Pre-Universe. (DWN: The Curse of Fenric) The parallel dimension that the Great Old Ones shunted themselves into, which collapsed seconds later into the current universe, may have been one of the Shadow Dimensions. (MA: Millennial Rites)

The Seventh Doctor claimed King Arthur's craft found in Lake Vortigern was from another dimension or parallel universe. (DW: Battlefield)

Anti-Time was a dimension created by the use of the Oubliette of Eternity; it was inhabited by the Neverpeople. (BFA: Neverland)

Jason Kane spent several years in a hell dimension. It was predominately rock. Tunnelling craft tunnelled their way between habitable areas. (BFBS: The Infernal Nexus)

Species from different dimensions

 * Transcendental Beings, who exist in the higher dimensions such as...
 * The Guardians of Time
 * The Chronovores
 * The Eternals
 * Spillagers (BFA: Winter for the Adept)
 * Seven-dimensional Legions (NA: Lucifer Rising)
 * Eight-dimensional The Scourge (BFA: The Shadow of the Scourge)
 * Good Daleks (BFDE: Project Infinity)
 * Neutron eaters (BBCR: Lost Souls)