nosing

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The horizontally projecting edge of a stair tread.
2. n. A shield covering this edge.
3. n. A projecting edge of a molding.

Century Dictionary and Cyclopedia

1. n. In architecture, the projecting edge of a molding or drip; the projecting molding on the edge of a step in a stair.
2. n. In a lock, the keeper which engages the latch or bolt.
3. n. A metal or rubber shield formed to fit the projecting edge of a tread or step of a stairway to protect it from wear. Such nosings are frequently extended to cover or partly cover the tread also, and roughened or embossed to prevent the feet from slipping upon them. Also called stair-nosing.

Wiktionary

1. v. present participle of nose.
2. n. That part of the treadboard of a stair that projects over the riser.
3. n. Any similar projection, such as the projecting edge of a moulding.

GNU Webster's 1913

1. n. (Arch.) That part of the treadboard of a stair which projects over the riser; hence, any like projection, as the projecting edge of a molding.