Definition of retentionist in US English:

retentionist

noun

  • 1Generally. A person who advocates the retention of something; a supporter of retention.

  • 2Chiefly historical. A person who advocates the retention of territory, especially the retention by the United States of the Philippines and other Spanish overseas territories annexed during the Spanish-American War (1898).

  • 3A person who advocates the retention of a law or practice; specifically a person who advocates the retention of capital (or occasionally corporal) punishment.

adjective

  • Of or relating to retentionists; that advocates retention of something.

Origin

Mid 19th century; earliest use found in The Zoologist: a miscellany of natural history. From retention + -ist.