相 means 'mutually' or 'each other', and 差 means 'difference' or 'to differ'. Together, 相差 expresses the idea of two or more things differing *by a specific amount* — usually in quantity, age, distance, time, or degree. It is almost always followed by a measure phrase (e.g., 三岁, 两公里, 十分钟), never used alone. Unlike the verb 'differ' in English, 相差 is not transitive and does not take an object marked with 把 or 被; it functions as a predicate describing a measurable gap between entities.

This word is common in comparisons involving concrete, quantifiable differences: ages of people, heights of buildings, temperatures on different days, or scores in a competition. It carries a neutral, factual tone and appears frequently in daily conversation, news reports, and data descriptions. While it can be used for abstract concepts like opinions or attitudes, those uses are rarer and often require additional context to sound natural.