A dill pickle is a cucumber preserved in a salty, vinegar- or brine-based pickling liquid flavored with dill, giving it a balance of salty, sour, and herbaceous flavor along with a crisp bite. Pickling cucumbers in an acidic brine is an ancient technique, reportedly practiced in Mesopotamia as early as 2400 BCE and later in Greece and Rome; dill was folded into the practice as it spread through Europe.
Dill pickles are a common condiment on hamburgers, hot dogs, and sandwiches in North America, a side dish or soup/salad ingredient in Eastern Europe, and comparable to (though distinct from) other pickled vegetables like Korean kimchi and Japanese tsukemono elsewhere in Asia. They pair well with rich or fatty foods - potato salad, deviled eggs, grilled burgers - and are a classic Bloody Mary garnish.