expression, noun
radiant smile; contented smile; beaming smile
Describes a smile that spreads across the whole face, expressing deep satisfaction or happiness. Often used in literary or descriptive contexts.
彼は満面の笑みで賞状を受け取った。
He accepted the certificate with a beaming smile.
孫の顔を見て、祖母は満面の笑みを浮かべた。
Seeing her grandchild's face, the grandmother broke into a radiant smile.
微笑み is a gentle, often subtle smile, whereas 満面の笑み is a broad, unmistakable smile.
The phrase combines 満面 (whole face) and 笑み (smile). The exact historical derivation is uncertain, but it is a natural collocation in Japanese.