Chado or Sado means "the way of tea;" it refers to tea ceremonies, or the study or doctrine of tea ceremonies. Read about it at Wikipedia. A single ceremony is called a chanoyu.
Chanoyu means "hot water for tea;" it refers to a very formal tea ceremony, in which maccha (see food page) is served. A chanoyu is a single ceremony, as opposed to the study of tea ceremonies, called chado. Read more about it here.
Comiket is short for "Comic Market," the world's largest comics convention, which is held twice a year in Tokyo. See the official website, Wikipedia, or Everything2.
Hara-kiri is the common term for ritual suicide in the form of self-disembowelment, more formally referred to as Seppuku (see lower this page).
Hatachi means "20 years old." This is the age of majority in Japan, when Japanese people can vote, smoke, drink, and marry without a parent's permission. There is a ceremony called "Seijin (or seinen) shiki," which happens on Seijin no hi (see festivals page), for everyone who will turn 20 during the current school year.
Jiang Hu Chinese underworld of like martial arts and gangsters and stuff, this segment of society is the setting for wuxia fiction (see arts page). See Jiang Hu: Chinese Martial Underworld, Wikipedia, or Everything2.
Kagami-wari Literally, "mirror-breaking," it actually refers to the sake barrel-breaking ceremony, which is usually done at a Shinto shrine for New Year's, for good luck in the coming year.
Karesansui or "dry landscape" gardens were originated in Japan centuries ago by Zen Buddhists, which is why Westerners commonly refer to them as "Zen gardens." They include kansho niwa, or "contemplation gardens," which are meant to be viewed from an attached building and never entered physically except by the gardener. They also include kaiyu-shiki-teien, or "stroll gardens," in which following a path will reveal a sequence of shifting scenes; and bonseki (see stuff page), which are miniature Zen gardens made on trays. Read more about Zen gardens at JGarden or Wikipedia.
Keitai The culture of cellphones (keitai denwa, see stuff page). Keitai in Japan are pretty much indispensible, even moreso than in the U.S., and are generally more advanced. Novel features for us are now commonplace in Japan, and many people practically couldn't- or at least wouldn't live without their keitai. It's a major cultural trend, bordering on addiction. And it's very cool. See Wikipedia or Everything2.
Ketsuekigata Blood type. Or the theory that blood type determines personality. The theory is so popular that anime or manga may mention characters' blood types. Blood type is also significant in other aspects of Japanese society, including dating. See Wikipedia, TV Tropes, or Everything2.
Kogal A Japanese subculture of girls who conspicuously display their disposable income, through fashions and such. Their money may come from parents or from dates (typically older men). See Wikipedia or Everything2.
Kyabakura Cabaret club; hostess (or host) club. Customers can drink as well as engage in conversation with hosts or hostesses. See Wikipedia, TV Tropes, or Everything2.
Mizu shobai "Water trade." Euphemism for night-time entertainment business in Japan, including host(ess) clubs, bars, etc. See Wikipedia.
Omiai A custom of matchmaking for possible marriage... similar to an arranged marriage, except a couple is under no actual obligation to marry. See Wikipedia.
Purikura "Print club," which is something like a photo booth. See Wikipedia.
Seppuku is the formal term for ritual suicide in the form of self-disembowelment, commonly called hara-kiri. See Wikipedia or TV Tropes.
Tokyo International Anime Fair is a major anime trade fair held annually in Tokyo. See official website or Wikipedia.
See also: arts & entertainments and clothing (under fashion trends).