A pickled cucumber (commonly known as a pickle in the United States and Canada, and a gherkin in Britain, Ireland, South Africa, Australia, and New Zealand) is a small or miniature cucumber that has been pickled in a brine, vinegar, or other solution and left to ferment for some time, by either immersing the cucumbers in an acidic solution or through souring by lacto-fermentation. Pickled cucumbers are often part of mixed pickles.
t is often claimed that pickled cucumbers were first developed for workers building the Great Wall of China,[1] though another hypothesis is that they were first made in the Tigris Valley of Mesopotamia, using cucumbers brought originally from India.[2]
Pickled cucumbers are highly popular in the United States and are a delicacy in northern and eastern Europe.[3] Pickled cucumbers are flavored differently in different regions of the world.[3]
Brined pickles are prepared using the traditional process of natural fermentation in a brine, making them grow sour.[4] The salt concentration in the brine can vary between 20–40 grams per litre (3+1⁄4–6+1⁄2 oz/imp gal; 2+3⁄4–5+1⁄4 oz/US gal). Vinegar is not needed in the brine of naturally fermented pickled cucumbers.

The fermentation process depends on the Lactobacillus bacteria that naturally occur on the skin of a growing cucumber. These may be removed during commercial harvesting and packing processes. Bacteria cultures can be reintroduced to the vegetables by adding already fermented foods such as yogurt or other fermented milk products, pieces of sourdough bread, or pickled vegetables such as sauerkraut.

Typically, small cucumbers are placed in a glass or ceramic vessel or a wooden barrel, together with various spices. Among those traditionally used in many recipes are garlic, horseradish, the whole dill stems with umbels and green seeds, white mustard seeds, grape, oak, cherry, blackcurrant and bay laurel leaves, dried allspice fruits, and—most importantly—salt. The container is then filled with cooled, boiled water and kept under a non-airtight cover (often cloth tied on with string or a rubber band) for several weeks, depending on taste and external temperature. Traditionally, stones (also sterilized by boiling) are placed on top of the cucumbers to keep them under the water. The cucumber's sourness depends on the amount of salt added (saltier cucumbers tend to be more sour).

Since brined pickles are produced without vinegar, a film of bacteria forms on top of the brine. This does not indicate that the pickles have spoiled, and the film may be removed. They do not keep as long as cucumbers that are pickled with vinegar and usually must be refrigerated. Some commercial manufacturers add vinegar as a preservative.