green tea vs herbal tea

green tea

noun
  • A beverage brewed from green tea leaves. 

  • Leaves of the tea plant which are treated with steam or roasted to denature the enzymes that cause the leaves to darken and change taste. (Without this step they would become oolong tea or black tea). 

herbal tea

noun
  • A beverage made by infusing the leaves of various herbs, generally not including true tea (Camellia sinensis). 

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