In the U.S., salad is nearly synonymous with lettuce.  It is a crucial ingredient and the first thing I add when making a salad. When I was abroad in Greece, I ordered a side-salad in a restaurant on one of my first days in Athens.  As the waitress brought out my food, I did a double-take after glancing at the salad.  To my surprise, there was no lettuce at all.  At first, I wondered if it was a specialty house salad but everywhere I visited in Greece served the same salad without lettuce.
  
During my experience, I learned that a typical Greek salad only consists of five main ingredients: tomatoes, cucumbers, onions, olives, and feta cheese.  Then you add a little bit of seasoning (usually salt and pepper) and some olive oil and some lemon juice.  It can be served in a bowl or even on skewers.  

So the next time you have a Greek salad with lettuce, know you are having an Americanized Greek salad.   At that point, why bother calling it a Greek salad?