shihakugan 四白眼

In Japanese, shihakugan 四白眼, meaning "four-white eyes," refers to having such small irises that they're surrounded by the white of the eye at all four sides—left, right, top, and bottom.

It's also read yonpakugan, and also known as shihaku eyes.

Endou Saya 遠藤サヤ, example of tiny irises, shihakugan 四白眼.

Character: Endou Saya 遠藤サヤ
Anime: Dagashikashi だがしかし (Episode 2)

Meaning

The term shihakugan is related to a more common term, sanpakugan 三白眼, which refers to having white on three sides surrounding your iris.

The basic idea is that, when you're looking straight forward, the number of sides around your iris that are white—in other words: that have visible "sclera," shirome 白目—determines whether it's shihakugan, sanpakugan, or just your normal, average eye.

Diagram showing the difference between sanpaku eyes, shita-sanpakugan 下三白眼, ue-sanpakugan 上三白眼, and shihakugan 四白眼.

Source: japanesewithanime.com (CC BY-SA 4.0)

Both terms can be used toward real people and anime characters alike.

Since sanpakugan is sometimes called "sanpaku eyes" in English, shihakugan would be "shihaku eyes."

One thing worth noting, though, is that by its definition, a character with large irises is technically shihakugan so long as their eyes are humongous enough to completely contain those large irises, plus some extra white space.

Gon Freecss ゴン＝フリークス, example of large irises at the middle of even larger eyes, shihakugan 四白眼.

Character: Gon Freecss ゴン＝フリークス
Anime: Hunter x Hunter (2011) (Episode 1)

Context: Gon has large but non-tangential irises.

However, when people use the term shihakugan in practice, it normally refers to a character that has extremely small irises instead.

Simon シモン, example of tiny irises, shihakugan 四白眼.

Character: Simon シモン
Anime: Tengen Toppa Gurren Lagann, 天元突破グレンラガン (Episode 3)

Sometimes, characters with shihakugan are thought to be staring or making an angry face at other characters.

Usui Kenta 雨水健太, example of character with small irises, shihakugan 四白眼.

Character: Usui Kenta 雨水健太
Anime: Karin かりん (Episode 1, Stitch)

Context: Kenta looks at a girl talking to him, who says:
nanimo niramanakutemo
何も睨まなくても
[You] don't need to stare at [me] [for this].
ore wa betsuni, me-tsuki φ warui no wa umare-tsuki dakara
俺は別に、目つき悪いのは生まれつきだから
[I'm not doing that, this is just my normal face.]
- betsuni - not particularly [doing it].
- me-tsuki ga warui - the look of one's eyes is bad, they look angry and unapproachable.
- umare-tsuki - attained when [I] was born, i.e. this is the face I was born with.