A noun meaning mirror or looking glass. In the ancient world, mirrors were made of polished bronze or silver and gave an imperfect, dim reflection compared to modern glass mirrors. Paul uses this as a key metaphor for the limitation of current spiritual knowledge compared to the face-to-face vision of God in eternity.
Paul's famous statement in 1 Corinthians 13:12 — 'now we see through a glass darkly, but then face to face' — uses esoptron to describe the partial, indirect nature of our current knowledge of God. Like looking into an ancient bronze mirror, our present knowledge reflects reality but imperfectly. This is simultaneously humbling (we don't have complete understanding) and hopeful (direct vision is coming). James uses the mirror differently — Scripture as a mirror showing us who we truly are, which we can glance at and forget, or study and be transformed by.