'Yī jīn' literally means 'one catty', a traditional Chinese unit of weight equal to exactly 500 grams (0.5 kg). The character 一 (yī) is the numeral 'one', and 斤 (jīn) is the unit itself—historically varying regionally but standardized in modern China as 500 g. Though metric units are official, 'jīn' remains widely used in daily life, especially in markets, kitchens, and informal speech when buying food, meat, vegetables, or live animals.

This measure is particularly common when discussing livestock or animal products—for example, pricing pork per jīn at wet markets, estimating feed portions for chickens or pigs, or describing the weight of a live fish or rabbit. It’s rarely used for manufactured goods or scientific contexts, where grams or kilograms dominate. Note that 'jīn' is not interchangeable with 'gōngjīn' (kilogram); one gōngjīn equals two jīn (1,000 g).