gulag vs penitentiary

gulag

noun
  • A prison camp, especially one used to hold political prisoners. 

  • Also GULAG: the system of all Soviet labour camps and prisons in use, especially during the Stalinist period (1930s–1950s). 

  • A place where, or political system in which, people with dissident views are routinely oppressed. 

verb
  • To compel (someone) into a forced labour camp or a similar place of confinement or exile. 

penitentiary

noun
  • A state or federal prison for convicted felons; (broadly) a prison. 

  • A priest in the Roman Catholic Church who administers the sacrament of penance. 

adj
  • Of or relating to penance; penitential. 

  • Of or relating to the punishment of criminals. 

How often have the words gulag and penitentiary occurred in a corpus of books? (source: Google Ngram Viewer )