sekigan 隻眼

In Japanese, sekigan 隻眼 means "one-eyed," in the sense that you've lost your other eye.

See tangan 単眼 for having a single eye like a cyclops.

Character: Hanzoumon Yuki 半蔵門雪
Anime: Release the Spyce (Episode 1)

• Meaning
• Styles
  
  • Visible
  • Scar
  • Eye Patch
  • Hair
• Canonicality

Meaning

The word sekigan 隻眼 has two morphemes:

• seki
  隻
  Lacking one of two parts.
• gan
  眼
  Eye.

It literally means lacking one of two eyes, in other words, it means having only one eye.

• kata-me wo ushinau
  片目を失う
  To lose one-of-the-two eyes.

For the sake of reference:

• ryou-me wo ushinau
  両目を失う
  To lose both eyes.
• moumoku
  盲目
  Blind.

The term sekigan pretty much always means having lost one eye, but in some situations it can mean something else. For example:

Character: Kaneki Ken 金木研
Anime: Tokyo Ghoul, Toukyou Guuru 東京喰種-トーキョーグール (Episode 1)

• In Tokyo Ghoul, ghouls' eyes can turn black, and the term sekigan is used toward ghouls that only have one ghoul eye, as opposed to two.

Styles

In manga and anime, there are various ways to express that a character has lost an eye.

Visible

The most basic way to show a character has lost an eye is just to visibly draw the eyeball-less eye socket.

This is some grotesque imagery, so it's unlikely to happen, and if it does happen, it's probably going to be censored with shadows or something anyway.

Character: Romero ロメロ
Anime: Zombieland Saga, ゾンビランドサガ (Episode 6)

Scar

Characters who have lost one eye in a sword fight are often drawn with a vertical slash scar across their eyes.

Character: Kubard クバード
Anime: Arslan Senki, アルスラーン戦記 (Episode 3)

• kizu-ato
  傷跡
  Scar.
  • kizu
    傷
    Injury.
  • ato
    跡
    Any trace or mark left by something.

Eye Patch

Most one-eyed characters wear an eye patch to cover their missing eye.

Character: Uryuu Minene 雨流みねね
Anime: Future Diary, Mirai Nikki 未来日記 (Episode 11)

• gantai
  眼帯
  Eye patch.
• houtai
  包帯
  Bandage.

Hair

In rare cases, a character whose hair always cover half of their face is actually missing one eye.

Character: Kitarou 鬼太郎
Anime: Gegege no Kitarou ゲゲゲの鬼太郎 (2018) (Episode 1)

• Canonically, Kitarou lost one eye when he was a child. There are variants of the story where he loses the right eye instead of the left.

Character: Ginko ギンコ
Anime: Mushishi 蟲師 (Episode 1)

Canonicality

In manga and anime, characters who apparently have only one eye sometimes turn out to have had two perfectly working eyes all along.

It's pretty much a trope at this point that characters introduced with eye patches, and that have constantly worn eye patches for many episodes, will eventually lift the eye patch to reveal a different-colored eye with some sort of ability hidden within.

Character: Hatake Kakashi はたけカカシ
Anime: Gekijouban Naruto: Dai Koufun! Mikazuki-jima no Animaru Panikku Datte ba yo!, 劇場版 NARUTO -ナルト- 大興奮！みかづき島のアニマル騒動だってばよ (Movie)

Besides these, there are even characters who wear eye patches because it looks cool.

Character: Megumin めぐみん
Anime: Kono Subarashii Sekai ni Shukufuku wo! この素晴らしい世界に祝福を！ (Episode 2)

Conclusion: just because a character LOOKS LIKE they only have one eye, doesn't mean they ACTUALLY have only one eye.

On the other hand, if a character is introduced with an eye patch in the first episode, and in the last episode they remove the eye patch, claiming the character isn't one-eyed is a spoiler to anyone who hasn't watched the whole thing.

If a list of one-eyed characters doesn't include a character, and a commenter asks "why isn't character X included?" The author of the list can't say: "well, it's because character X actually turns out to have two eyes in episode 404."

Consequently, in both English and in Japanese, one-eyed refers to characters that appear to be one-eyed when they're introduced, rather than characters that are canonically one-eyed.

Character: Nico ニコ
Anime: Nanbaka ナンバカ (Episode 1)

• Until he removes those bandages, Nico's right eye both is there and isn't there, as far as I'm concerned.