Retirement was the action of ceasing to work in a professional capacity or the state of having done so.
The Doctor
The Sixth Doctor once retired. (PROSE: The Spindle of Necessity)
Following the loss of Amy Pond and Rory Williams (TV: The Angels Take Manhattan) and narrowly averting disaster after being manipulated by the Daleks, (PROSE: The Dalek Generation) the Eleventh Doctor retired from his travels to Victorian London, secluding himself in his TARDIS for a lengthy period of time. (TV: The Snowmen, The Great Detective) He eventually returned to action after being phoned by Clara Oswald. (TV: The Bells of Saint John)
Later, the Eleventh Doctor voiced at the National Gallery his hope that he could retire one day and become the curator of it. Immediately after, he was met by the Curator himself, who implied that the Doctor would indeed become him one day. (TV: The Day of the Doctor)
The Twelfth Doctor promised Nardole to semi-retire from leaving Earth to guard the Vault at St Luke's University. Upon meeting Bill Potts though, he struggled to keep this promise. (TV: The Pilot) He eventually broke it completely. (TV: Oxygen, World Enough and Time)
Humans
As the oldest-serving staff member by several centuries, Jack Harkness' Torchwood pension would be massive, should he ever retire. (AUDIO: The Torchwood Archive)
Jeremiah O'Kane retired as director of Hawthorne and left his successor, Colin Dove, with a lifetime supply of tea. (HOMEVID: The Zero Imperative)
Whittaker was headmaster of Winterborne School for decades before the governors persuaded him to retire, replacing him with the younger Gavin Purcell. (HOMEVID: The Devil of Winterborne)
Dave Cooke retired as a cleaner at Torchwood Tower and was given a cake and engraved pen by Yvonne Hartman. (AUDIO: Locker 15)
Tommy Pierce wished to retire after fifty years of working at Torchwood One. However, Yvonne told him that he had agreed to work at Torchwood for life. (AUDIO: Retirement Plan)
Anne Hartman retired from the Land Registry due to her increased workload and depression. She moved to a small cottage in Shropshire. (AUDIO: The Rockery)
Graham O'Brien was a bus driver, but had retired prior to becoming a companion of the Thirteenth Doctor. (TV: The Woman Who Fell to Earth, Rosa)
In an alternate universe, Alistair Gordon Lethbridge-Stewart was disgraced and removed from UNIT. He retired to Hong Kong. (AUDIO: Sympathy for the Devil)
Other species
Prentice retired from adventuring to Keelmouth in 1933. (PROSE: Bide-a-Wee)