penal colony vs prison

penal colony

noun
  • A colonial territory used (mainly) for the detention and forced labor of deportees, typically where free labor is desperately scarce. 

prison

noun
  • A place or institution where people are held against their will, especially for long-term confinement of those awaiting trial or convicted of serious crimes or otherwise considered undesirable by the government. 

  • Confinement in prison. 

  • Any restrictive environment, such as a harsh academy or home. 

verb
  • To imprison. 

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