Mastiha (pronounced: mas-tee-ka) is a natural resin that comes from the Mastic tree (basically an evergreen shrub that was first cultivated on the Greek Island of Chios). People in ancient eastern Mediterranean countries commonly chewed (masticated) it to clean their teeth and freshen their breath. Today it can still be found in commercial chewing gums (among other foods) from Greece and other Eastern Mediterranean countries.

I was so surprised to find modern mastiha gum at the grocery store in Athens.

I learned about this fact from: The ELMA gum box.