A position of lying or facing upward (on one's back); usually used as '〜に仰向け(に)'.
赤ちゃんは仰向けに寝かせると息がしやすいと言われている。
It's said that it's easier for babies to breathe when laid on their backs.
検査のためベッドで仰向けになってください。
Please lie on the bed on your back for the examination.
彼は仰向けで星空を眺めていた。
He was lying on his back looking at the starry sky.