A Tread

1. A step taken with the foot.
2. A manner of stepping.
3. The sound made when someone or something is walking.
4. (obsolete) A way; a track or path.
5. The horizontal part of a step in a flight of stairs. Stair tread - Wikipedia
6. The grooves carved into the face of a tire, used to give the tire traction. [from 1900s] Tire tread - Wikipedia
7. The grooves on the bottom of a shoe or other footwear, used to give grip or traction.
8. (biology) The chalaza of a bird's egg; the treadle.
9. The act of avian copulation in which the male bird mounts the female by standing on her back.
10. (fortification) The top of the banquette, on which soldiers stand to fire over the parapet.
11. A bruise or abrasion produced on the foot or ankle of a horse that interferes, or strikes its feet together.

(horizontal part of a step): run

(horizontal part of a step): rise, riser