First Floor, Floor One, Ground Floor, First Story
noun
The one floor is the first floor (or floor one). In Japan, that means the ground floor. Pretty simple.
いっかい
The reading is on'yomi because it's a jukugo kanji. Be careful of the 一 though. It gets shortened and has a little っ in there.
一階にはお店がたくさんあります。
There are many shops on the first floor.
このフロアのクリーニングが終わったら、一階に来てくれる？
After cleaning this floor, will you come to the first floor?
トイレは一階の出口の横にあります。
The bathroom is next to the exit on the first floor.