Definition of riser noun from the Oxford Advanced American Dictionary
riser
noun
/ˈraɪzər/
1early/late risera person who usually gets out of bed early/late in the morning2(technical)the vertical part between two steps in a set of stairs compare tread3risers[plural]a set of wide steps that have several levels and are intended for performers to stand onThe chorus went up to the stage and stood on the risers.