A bare face with no make-up; the unadorned face. Often used interchangeably with 'すっぴん' in everyday speech.
今日は疲れているから素顔で出かける。
I'm tired today, so I'll go out without makeup.
寝起きの素顔を見られるのはちょっと恥ずかしい。
It's a bit embarrassing to have someone see my face right after waking up.
彼女は普段から素顔がきれいだと言われる。
People say her bare face is beautiful even normally.
Figuratively, it means the true face or real nature of something (e.g., a country, celebrity) — the reality behind appearances.
そのドキュメンタリーで政治家の素顔が明かされた。
The documentary revealed the politician's true nature.
表向きは優しいが、素顔は冷酷だと噂されている。
On the surface he's kind, but his real nature is rumored to be cold-hearted.
メディアが伝えるイメージと企業の素顔は違っていた。
The image presented by the media differed from the company's true nature.
Can mean a sober or composed expression — the calm, unadorned look someone has (context-dependent).
重大な発表でも、彼女は素顔で淡々と対応した。
Even during the major announcement, she responded calmly with a composed face.