Tread

Stair Tread

The Tread is the top or horizontal surface of a step i.e. the part of the stairway that is stepped on. The tread "depth" is measured from the outer edge of the step from edge of the nosing to the vertical "riser" between steps. The "width" is measured from one side to the other.

WIKIPEDIA ENTRY FOR STAIRWAY.

Tread Picture    Tread 2 Picture