A stair is one of the flat steps you climb to reach a higher floor. The bottom stair, sit on the stair, a spiral stair.
Every step in a flight must be the same height, because the foot learns the rhythm after two or three and a single odd one causes falls — building regulations set this tightly. The singular is rarer than the plural. Note staircase and stairwell.
1nounstairOne of the flat steps climbed to reach a higher floor.
“She sat on the bottom stair to tie her shoes.”
stair is a step.
Old English staeger names a stair, from a Germanic root meaning to climb. Stile and stirrup are distant relatives.
Middle English
steire
English, today
stair
From Middle English steire. From Middle English steire, staire, stayre, stayer, steir, steyre, steyer, from Old English stǣġer (“stair, staircase”), from Proto-West Germanic staigri, from Proto-Germanic staigriz (“stairs, scaffolding”), from Proto-Indo-European steygʰ- (“to walk, proceed, march, climb”). Cognate with Dutch steiger (“a stair, step, wharf, pier, scaffolding”), Middle Low German steiger, steir (“scaffolding”), German Low German Steiger (“a scaffold; trestle”). Related to Old English āstǣġan (“to ascend, go up, embark”), Old English stīġan (“to go, move, reach; ascend, mount, go up, spring up, rise; scale”), German Stiege (“a flight of stairs”). More at sty.
Etymology from Wiktionary, CC BY-SA