階|かいkai[1]atamadaka (head-high)

1. noun, noun (suffix)storey; story; floor
   それはもう一ひとつ下もとの階かいですよ。↗

   That's one level down.
2. nounstairs
3. noun, noun (suffix)stage (in chronostratigraphy)
4. countercounter for storeys and floors of a building
   私の部屋へやは４階かいにあります。↗

   My apartment is on the fourth floor.

• SynonymフロアNeutralLoanword for a floor or floor space, including a shop's sales floor, as in 「3階フロア」; it stresses the flat area, while 「階」 mostly counts storeys.
• RelatedレベルNeutralLoanword for level or grade, stressing how high or low something stands; it does not refer to a building's storeys.
• Related階層かいそうWrittenA written-style word for social strata or hierarchical layers, as in 「富裕層」; abstract, and not used to count a building's storeys.
• Related上がり段あがりだんNeutralSteps at a building's entrance, leading up from the ground to the floor; confined to entryways and not often used.
• Related階梯かいていWrittenA written word for a staircase or, figuratively, an introductory guide; rarely seen in modern Japanese.
• Related梯子段はしごだんNeutralThe steps of a staircase, with an old-fashioned flavor; in everyday speech people say 「階段」.

Use 「階」 for a building's storeys, as in 「3階」. Choose 「フロア」 for floor space or a sales floor, 「レベル」 for levels of ability or rank, 「階層」 for social strata, and 「梯子段」 for steps; 「階梯」 is rare.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. （カイ）多数たすうの層そうからなる建築物けんちくぶつにおいてそれぞれの層そうもこと。
2. （カイ）階段かいだんのこと。きざはし、きだはし。
3. （カイ）官吏かんりの序列じょれつ。位階いかい。
4. （カイ）建物たてものの層そうを数かぞえる助数詞じょすうし。
5. （カイ） 関数かんすうを繰り返くりかえし微分びぶんするときの、微分びぶんした回数かいすうの助数詞じょすうし。