A generation was a group of people born around the same time, and thus living in roughly the same age group in the same time period.
The "Dalek Generation" was a name used to describe the people born during the Second Dalek War, who didn't know what it was like to live in a time of peace. (PROSE: Prisoner of the Daleks).
The Fifth Doctor speculated that a generation was every twenty-five years. (TV: Four to Doomsday)
The Fourth Doctor claimed that for a thousand generations, the name "Dalek" would spread fear and terror across the universe. (TV: Genesis of the Daleks)
Vaber recalled that the Thals' war with the Daleks on Skaro took place generations prior to 2540. (TV: Planet of the Daleks)
The Disciples of Omega took four generations to install the transduction barriers on Gallifrey. (AUDIO: Renaissance)
The Tenth Doctor termed the combatants in World War I as the "lost generation", since the lives of so many young men were cut short. As a result, the Weeping Angels feeding at this time were not satisfied by their potential energy. (COMIC: The Weeping Angels of Mons)
According to Rhys Williams, Kaitlin Russell belonged to a generation of millennials, who placed an emphasis on the Earth's environment. (AUDIO: Sargasso)