累积 (lěi jī) is a verb meaning 'to accumulate' — emphasizing a gradual, step-by-step buildup over time, often of something measurable or intangible like experience, knowledge, wealth, or evidence. The character 累 (lěi) conveys the idea of repeated action or layering, while 积 (jī) means 'to pile up' or 'to store', reinforcing the sense of steady accumulation. Together, they form a compound that stresses continuity and incrementality, not sudden increase.

This word is commonly used in formal, academic, or professional contexts — for instance, when discussing data trends, learning outcomes, environmental changes, or financial growth. It frequently appears with time-related phrases like '多年来' (over many years), '逐渐' (gradually), or '不断' (continuously). Unlike the more colloquial 积累 (jī lěi), which shares similar meaning but has broader register usage, 累积 carries a slightly more technical or statistical nuance and is often found in written reports, scientific descriptions, or policy documents.