The first floor of a building (ground floor).
店はビルの一階にあります。
The shop is on the first (ground) floor of the building.
一階で受付を済ませてください。
Please complete reception on the first floor.
One floor or a single storey — used to indicate a building has one level or to count floors.
この家は一階建てで、階段がない。
This house is single‑storey and has no stairs.
会議は一階の広いフロアで行われる予定だ。
The meeting is scheduled to take place on a large single floor.
Can mean the first rank or first level in a graded system; refers to the top or initial tier in a hierarchy depending on context.
この制度では一階が最も基本的なランクだ。
In this system, the first rank is the most basic level.