I wasn't aware the Japanese had a particular name for this phenomenon, but I found this entry on wikipedia:
For the real budget traveller wanting to get by on the cheap in Japan is the option of nojuku (野宿). This is Japanese for "sleeping outside", and although it may seem quite strange to westerners, a lot of young Japanese do this when they travel. Thanks to a low crime rate and relatively stable climate, nojuku is a genuinely viable option if you're travelling in a group or feel confident doing it on your own. Common nojuku places include train stations, michi no eki (road service stations), or basically anywhere that has some kind of shelter and public toilets nearby.
Those worrying about shower facilities will be delighted to know that Japan is blessed with cheap public facilities pretty much everywhere - notably onsen, or hot springs. Even if you can't find an onsen, sento (public baths), or sauna are also an option.
Bear in mind nojuku is only really viable in the summer months, although in the northern island of Hokkaido even in summer the temperature may dip during the night. On the other hand, there's much more scope for nojuku on Okinawa (although public facilities on the smaller islands are lacking).
Nojuku is not really recommended for first-time travellers to Japan, but for those with some experience, it can be a great way to get into the 'onsen' culture, meet other fellow nojuku travellers, and most of all travel very cheaply when coupled with hitchhiking.
The season for nojuku is fast approaching with the imminent arrival of cherry blossoms and the changing temperatures. I may have to take advantage of sleeping outside when I'm backpacking around Shikoku...
In other news, the Onsen Matsuri will be in Beppu next weekend. Never pass up an opportunity to attend a Japanese matsuri, especially one centered around the bathing culture. With Beppu accessible by train and ferry, you have no excuse. Apparently they use special lights to make the steam in the Kannawa area multicolored: