shopping center vs strip mall

shopping center

noun
  • An area, either enclosed or unenclosed, hosting multiple retail establishments; a shopping mall. 

strip mall

noun
  • A shopping center consisting of a row of stores with separate entrances from the outdoors, typically along a road and/or with a parking lot in front. 

How often have the words shopping center and strip mall occurred in a corpus of books? (source: Google Ngram Viewer )