An ingredient might be exotic by my definition if: 1) I cannot find it in my local major grocery store chain; 2) I cannot find it anywhere in my city (including the organic/health food grocery, the local food co-op, the ethnic-specific grocery stores, and the local spice shop. Another criteria is how frequently/rare an ingredient is found in a all-recipes search of EYB, because EYB is so extensive.
I find fruits and vegetables to be more difficult to find (i.e. more exotic) than spices, sauces, and long-lasting ingredients (such as grains, canned/jarred/frozen products) -- probably because of the problems in shipping and preservation. There are several vegetables that I have seen in my cookbooks, but never in reality.