The vertical part of a stair step, or a person or thing that rises or gets up.
From 'rise' (Old English 'rīsan', from Proto-Germanic) plus the agent suffix '-er' (one who does). In architecture, 'riser' has been used since the 15th century for the vertical stair component.
Early risers, midnight risers, and stair risers—the same word works for all because it captures the essential idea of something going upward! This efficiency is why English speakers use 'riser' so naturally across different contexts.
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