Boob Window

In anime, when a girl's clothes for some reason has a hole in the middle of the chest that lets show her breasts, the term for that in Japanese is tanima hooru 谷間ホール, literally "cleavage hole." In English, it's usually called "cleavage window," or "boob window."

A single word term for it is "keyhole." Sometimes, it's also called a "peekaboo."

Character: Kanroji Mitsuri 甘露寺蜜璃
Anime: Kimetsu no Yaiba 鬼滅の刃 (Episode 22, After-Credits Scene)

• Examples
  • China Dress
• References

Examples

For reference, some examples:

Character: Pirotessa ピロテース
Anime: Lodoss-tou Senki, ロードス島戦記 (Episode 9)

• Context: a dark elf, daaku erufu ダークエルフ.
• erufu-mimi
  エルフ耳
  Elf ears.

Left: Shera L. Greenwood シェラ・L・グリーンウッド
Right: Rem Galeu レム・ガレウ
Anime: Isekai Maou to Shoukan Shoujo no Dorei Majutsu 異世界魔王と召喚少女の奴隷魔術 (Episode 1)

• Context: an elf next to a character with "cat ears," nekomimi 猫耳.

Character: Toa トア
Anime: Tsuki ga Michibiku Isekai Douchuu 月が導く異世界道中 (Episode 5)

• Context: bare shoulder are sometimes called soutai ryouiki 相対領域 "relative territory," due to bare thighs being called zettai ryouiki 絶対領域, "absolute territory."
• See also: soap bubbles background effect.

China Dress

One type of clothing that often features a cleavage hole is a "china dress," in Japanese: chaina-doresu チャイナドレス, whose technical name in English is "cheongsam," from the Chinese word chángshān 長衫.^{(en.wiktionary.org:長衫)}

Character: Konoe Subaru 近衛スバル
Anime: Mayo Chiki! まよチキ！ (Episode 6, Stitch)

Left: Mayonaka Matome 万世架まとめ
Middle: Arisa Ahokainen 有紗・アホカイネン
Right: Denkigai Niwaka 伝木凱にわか
Anime: Akiba's Trip The Animation (Episode 2)

Character: Lei-Lei, レイレイ, Hsien-Ko
Anime: Vampire Hunter (1997, OVA) (Episode 2, Collage)

• moe-sode
  萌え袖
  A term for overly long sleeves that cover one's hands, or, in this case, claws..

References

• 谷間ホール - dic.pixiv.net, accessed 2020-01-19.
• Cleavage Window - tvtropes.org, accessed 2020-01-19.
• 長衫 - en.wiktionary.org, accessed 2021-06-09.