always; constantly; in all circumstances
Describes unwavering, often quiet support or presence; literally 'in shadow and in sunlight', implying through good times and bad. Often used for someone who helps or watches over another without fail.
She has supported him through thick and thin.
My parents are always wishing for my success, no matter what.
常に means 'constantly' in a more neutral or formal sense; 陰に日に carries a warmer, more personal nuance of steadfastness.
Standard kanji spelling; 陰 suggests shade or hidden support, fitting the nuance of constant, behind-the-scenes care.
Alternate spelling using 影 (shadow, figure); less common but still encountered.
Compound of 陰 (shade, hidden side) + に + 日 (sun, day) + に, literally 'in the shade and in the sun', implying constancy across all conditions. The exact historical origin is uncertain, but the phrase is used idiomatically in modern Japanese.