Pronunciation: [dˈa͡ʊnste͡ə] (IPA) 
The correct spelling of the word for the lower floor of a building is "downstairs" /daʊnˈstɛrz/. The word is made up of the prefix "down" and the word "stairs". The "s" in "stairs" becomes a "z" sound in the suffix "-s" due to the voicing rule. The spelling "downstair" is incorrect and may be a common misspelling. It is important to use proper spelling in written communication to avoid confusion and errors.
Downstair is an adjective that describes something located at a lower level of a building or structure, specifically on a lower floor relative to one's current position. It denotes an item, room, or area within a multi-story structure that is situated beneath or lower than the location of the speaker or the point of reference being discussed.
In its plural form, "downstairs," the word refers to the lower floors of a building or the downward direction leading to lower levels. Downstairs commonly refers to spaces such as basements, lower levels, or ground floors.
The term is often used interchangeably with "lower floor" or "lower level," with the distinction that it emphasizes the vertical downward direction in relation to the point of reference. For instance, "Please go to the downstair office." In this context, "downstair" signifies that the office is located on a floor below the person giving the direction.
Furthermore, "downstair" can also be used as an adverb to describe the manner in which one moves or goes from a higher level to a lower level. For example, "She slowly walked downstair, cautiously descending each step."
In conclusion, downstair refers to something situated at a lower level within a building or structure relative to the speaker or reference point. It signifies a downward direction and can be used as both an adjective and an adverb to describe location or movement.
The word "downstair" is formed by combining the prefix "down-" (indicating a lower position or direction) with the noun "stair" (referring to a step or flight of steps). The etymology of "stair" can be traced back to the Old English word "stæger", which ultimately comes from the Proto-Germanic "stigraz". This term is related to the Old Norse "stiga" and the Old High German "stigan", all of which mean "to climb" or "to ascend". The addition of the prefix "down-" creates the compound word "downstair", specifically used to describe the lower level or floor of a building accessed by stairs.
