Rel
A rel was a Dalek unit of measurement, principally of time.
The Daleks used rels for time measurement (AUDIO: The Genocide Machine, The Mutant Phase, TV: Doomsday), as had the Kaleds before them (AUDIO: Guilt) A rel was slightly longer than a second. (TV: Evolution of the Daleks)
In other contexts, it was a unit of speed. (COMIC: Plague of Death, The Emissaries of Jevo) A rel was also used as a unit of hydroelectric energy, not to be confused for a vep, the unit used for artificial sunlight. (The Dalek Book) When the New Dalek Empire tried to destroy the universe with the reality bomb using the Medusa Cascade, they counted down from ten rels until their attempt to use it. (TV: Journey's End)
Behind the scenes
The word rel first appeared in the non-canonical movie Daleks' Invasion Earth 2150 A.D. before appearing in early Doctor Who comics.