1To become narrower or thinner (in width, scope, or e.g. eyes becoming narrow); an intransitive verb meaning 'to narrow'.
彼は冗談を聞いて目が細まった。
He smiled (his eyes narrowed) when he heard the joke.
道が次第に細まって、一台しか車が通れなくなった。
The road gradually narrowed until only one car could pass.
列は先端に向かって細まっていった。
The line tapered (narrowed) toward the front.