Part of Speech: noun

Definition: a soft luster on a surface.

Source: Words at Dictionary

Discovering colors in Prison by Nancy Gomez, line 9

“The cold, tensile sheen/ that buzzes off the electrified fence, the color of barbs/ in barbed wire.”

The word “Sheen” describes a lustered surface to feel soft. The author is describing the electric fence and the barb wires in the poem.