STAIR

7

• stair
• 1

  stairs

  [plural] a set of steps built for going from one level of a building to another [↪ upstairs,downstairs]
• up/down the stairs
• Jerry ran up the stairs.
• the top/head of the stairs
• I left my briefcase at the top of the stairs.
• the bottom/foot of the stairs
• 'Lisa,' he cried from the foot of the stairs.
• We walked up four flights of stairs (=sets of stairs).
• 2 [countable] one of the steps in a set of stairs:
• Lucy sat down on the bottom stair.
• 3 [singular] literary a staircase
• 4

  below stairs

  British English old-fashioned in the servants' part of a large house, in the past

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.