A Chelsea pensioner is an in-pensioner at the Royal Hospital Chelsea, a retirement home and nursing home for former members of the British Army located in Chelsea, London. Historically, however, the phrase applied more widely, referring to both in-pensioners and out-pensioners.
Read more about Chelsea Pensioner: In- and Out-pensioners, Life of In-pensioners, Men in Scarlet Album, Conditions For Admission As An In-pensioner, Admission of Women, Clothing, Chelsea Football Club