Pickled daikon. Full name "Takuanzuke." たくあん、沢庵、（漬）。
The pickling process used is nukazuke, where vegetables are for several months buried in a bed of the rice bran (nukadoko). In the past, Japanese farmhouses all had their own nukadoko and daughters would get the nuka culture from their mothers.
[Takuan. Photo Ad Blankestijn]
Daikon radish (or rettich) is the most popular vegetable for making nuka pickles.
First, the daikon radishes are hung in the sun for a few weeks until they become flexible. Next, they are placed in a pickling crock and covered with a mix of salt and rice bran. Optional are sugar, daikon greens, konbu, persimmon peels and chili pepper. A weight is then placed on top of the crock, and the radishes are allowed to pickle for several months.
Takuan made in this traditional way is only slightly, whitish yellow. To make the product look more attractive, the radishes are often colored yellow by adding turmeric (ukon), which is still a natural ingredient, but most mass-produced takuan rely on food coloring for the same effect.
Supermarket, cheap takuan can be terrible (tough and tasteless). But good takuan by a small dedicated producer is delicious! Look for takuan that is not too brightly yellow - some takuan is sold whole and with some nuka still on it - that is usually a sign of good quality.
Takuan, by the way, is in fact the name of a famous Zen priest (Takuan Soho, 1573-1645) who purportedly invented this type of pickle.
[This post has reworked a few elements from Wikipedia]