mart vs megastore

mart

noun
  • A shop, store. 

  • Marque (chiefly used in the phrase letters of mart). 

  • A bazaar, fair, marketplace. 

  • Salt beef. 

  • A head of feeder cattle or fattened cattle (usually the latter). 

megastore

noun
  • A large supermarket or similar retail outlet; a superstore. 

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