Mastiha is a Greek liqueur made from mastic gum shown on right in above picture. It has a unique flavor. The two closest common flavors that I can think of are banana and pink bubblegum, but it is as far from these flavors as they are from each other.
Mastic gum flavoring is useful in sweet pastry and cookies because it accentuates the flavor of sweet dough. People have described cookies flavored with mastiha as having extra doughy flavor.
This stuff is getting harder and harder to buy. Chicago had a small Greek neighborhood near my house, which is now gone. To replenish my stock today, I had to drive into Greektown where it takes half an hour to find parking. The Athens Grocery Store only had one bottle left. Incidentally, it is not expensive at $13.99, but Chicago sales tax is 9%.
Mastic essential oil exists, but all my sources are out of stock.