Federal judges on senior status
Senior status is a classification for federal judges at all levels who are semi-retired.
Qualifications for senior status
There is a commonly known rule known as the "Rule of 80" which is the shorthand method for the age and service requirement for a judge to assume senior status, as set forth in Title 28 of the US. Code, Section 371(c).
Beginning at age 65, a judge may retire at his or her current salary or take senior status after performing 15 years of active service as an Article III or Federal Appeals judge (65+15 = 80). A sliding scale of increasing age and decreasing service results in eligibility for retirement compensation at age 70 with a minimum of 10 years of service (70+10=80). Senior judges, who essentially provide volunteer service to the courts, typically handle about 15 percent of the federal courts' workload annually. 
Federal judges are eligible for senior status at the following ages:
Retired bankruptcy, magistrate, and Court of Federal Claims judges who continue to serve are called recalled judges, rather than senior judges.
History of senior status
In 1919, Congress created the senior status option for inferior court judges. Before the senior status option was created, a judge who reached the age of seventy with at least ten years of service as a federal judge was allowed to retire and receive a pension for the rest of his life; afterwards, a judge who qualified for retirement could instead assume senior status. In 1937, the option was extended to Supreme Court justices, although justices so electing are generally referred to as "retired" justices rather than as on senior status. A senior justice is essentially an at-large senior judge, able to be assigned to any inferior federal court by the Chief Justice, but receiving the salary of a retired justice. However, a retired justice no longer participates in the work of the Supreme Court itself.
In 1954, Congress modified entry requirements for the senior status option. Federal judges or justices could still assume senior status at seventy with ten years of service, but they could also assume senior status at 65 with fifteen years of service. In 1984, the requirements were further modified to what is often called the “Rule of 80”: once a judge or justice reached age 65, any combination of years of age and years of service on the federal bench which totaled to eighty entitled the judge to assume senior status.
Senior status was not originally known as such. In 1919, when the senior status option was created, a judge who had assumed what we now call senior status was referred to as a “retired judge”. The title of “senior judge” was instead used to refer to the active judge with the most seniority in a given court; after 1948, this notion was formalized and modified under the new title of Chief Judge. In 1958, the term Senior Judge was given its current meaning of a judge who had assumed senior status.