'Xíng yǐng bù lí' literally means 'form and shadow never part' — evoking the image of a person and their shadow, which naturally stay together under light. This idiom describes an extremely close, inseparable relationship between two people (or sometimes between a person and an object or habit), suggesting constant companionship, mutual dependence, or deep emotional bonding.

It’s commonly used to describe best friends, romantic partners, devoted siblings, or even a child and their favorite toy. While poetic and vivid, it carries a warm, positive connotation and is often found in storytelling, descriptive writing, or affectionate speech. Unlike neutral terms like 'together', 形影不离 implies emotional closeness and near-constant physical proximity — not just convenience or coincidence.