What does 日常 Nichijou mean?

Nichijou is the everyday: the ordinary, repeating run of life, as against anything exceptional. On its own it is plain vocabulary, and it becomes a category when 系 (kei, type) is attached to it.

The characters

日 (nichi) is day or sun. 常 (jou) is the constant, the usual, the always-so. The pairing gives the daily-constant, which is a more precise idea than English everyday, since it emphasises repetition rather than mere ordinariness.

The word has a natural opposite, the non-everyday, and Japanese uses that pairing readily. Fiction is often discussed in terms of which of the two a work is interested in.

The category

Everyday-type fiction concerns people to whom nothing much happens. Friends in a classroom, a family across a year, colleagues in a small shop. There is no threat to be resolved and often no arc in the usual sense, and a summary of the plot is unavoidably a list of small events.

English translates this as slice of life, which is close and carries an unhelpful implication: a slice suggests a sample taken from something larger and more eventful. The Japanese label makes no such promise. The everyday is not an excerpt from the story, it is the story.

Why nothing happening is a design

Because the effects available in that mode are unavailable elsewhere. Repetition can make a small change enormous. A character who has sat in the same seat for twenty episodes moving to a different one carries weight that no amount of plot could manufacture, and it works only because the show spent twenty episodes establishing the seat.

Read as a story with the incidents removed, this is empty. Read as a form that trades event for accumulation, it is doing something drama cannot.

How it differs from iyashikei

The two overlap heavily and are not the same. This one describes content: ordinary lives, low stakes. Iyashikei describes effect: the work leaves the viewer calmer.

A great deal of everyday-type fiction is soothing, which is why English collapses them. But a work can be relentlessly ordinary and quietly unbearable, following someone whose everyday is the problem, and it remains one category and not the other.