One of my (Clay) favorite traditional Japanese food is umeboshi. 

梅干(うめぼし)

梅 ume plum + 干 hoshi (boshi with phonetic change after ume) dry = pickled dried plum

This is also written with the 送(おく)り仮名(がな) (hiragana that follows the kanji to complete the reading):

梅干(うめぼ)し

梅干(うめぼし) are usually roundish, red, wrinkled and about the size of a large grape. But there are many varieties found throughout Japan. Most are salty and very sour but some are a little sweet. It is often said to be very healthy, but it must have a very high amount of sodium.

The above photo are the last two 梅干(うめぼし) in the Boutwell house. Yumi's mother will get more in the mail, but I'm afraid we will be 梅干(うめぼし)-less for a few weeks. The dark clump to the right is called 紫蘇(しそ). It is from a plant (perilla; a beefsteak plant) and helps give a consistent and dark coloration. It is very tasty with umeboshi and rice.

Usually umeboshi is eaten with rice. A cup full of rice only needs one umeboshi to be full of flavor.

When one has a cold, thin, almost soup-like rice is served with umeboshi. This is called お粥.(かゆ)

A common 弁当(べんとう) lunch box is called 日(ひ)の丸弁当(まるべんとう) which resembles the flag of Japan (a single 梅干(うめぼし) in the middle surrounded by white rice).

But perhaps most common is for 梅干(うめぼし) to be found in おにぎり. おにぎり are rice balls often wrapped in のり (seaweed). Pitted 梅干(うめぼし) can be placed in the center of the おにぎり for a very scrumptious snack.

You can buy 梅干飴(うめぼしあめ) (Umeboshi candy) and soda made with umeboshi juice (probably all artificial flavors but it is good!)

A not so nice word is 梅干婆(うめぼしばばあ). I'm sure you can figure out what context it is used, but it means a prune-faced old hag. I don't suggest using it. Ever.