A gimlet, an old fashioned cocktail that a lot of bartenders have never even heard of before. Its a fun litmus test of the knowledge and skill of the bartender, and really tasty and refreshing if made right.
Its just gin and lime syrup, about a 2 or 3 to 1 ratio, shaken with with lots of ice and served neat.
I think its pretty standard stock in a bar, I’ve never had someone not have it. There are other drinks with lime flavor too.