The preserved radish sometimes added to Pad Thai is called chai po. The ingredient is a salted or dried-preserved white radish, usually daikon, chopped into small pieces before being stir-fried with the noodles.
Chai po contributes a slightly sweet, salty, earthy flavor and a chewy texture. In Thai cooking it may also be called hua chai po, while related Chinese names include chai poh or chai por. The spelling varies because the name is transliterated from Asian languages, but “chai po” is a recognizable canonical answer.
Preserved radish is an optional Pad Thai ingredient rather than a universal component. Recipes may include it alongside rice noodles, tofu, egg, bean sprouts, shrimp, dried shrimp, peanuts, and chives. It is not the same as kimchi or pickled ginger: those are different preserved foods with distinct flavors and culinary traditions.