It refers to the vegetable, usually parsley or celery, that is dipped in liquid (usually salt water) and eaten.
en.wikipedia.org These include other vegetables such as squash, melons, peppers, beans, tomatoes, carrots, celery, lettuce, spinach and beets, various weeds and many ornamentals and bedding plants.
en.wikipedia.org The most common ingredients are scallops, peapods, boneless fish fillet, celery sticks, straw mushrooms, calamari, shrimp.
en.wikipedia.org Common ingredients include potatoes, onions, celery, corn, carrots, and bacon.
en.wikipedia.org