Small cucumbers pickled in brine or vinegar, often with dill: sour, salty and crunchy, sold in jars. They are eaten as a side and sliced into burgers, sandwiches and salads.
酸黄瓜 refers specifically to Western-style pickled cucumbers. Chinese fermented cucumbers (泡黄瓜) are a different product. The word entered Chinese mainly through exposure to fast food and Western cuisine.