日活 is the standard short form for 日活跃用户 (DAU). The full trio in Chinese tech-investor speak: 日活 (DAU) / 月活 (MAU, monthly active users) / 周活 (WAU, weekly). Investors quote them constantly when valuing apps like 抖音, 微信, and 小红书.
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Internet-industry jargon. Common in product reviews, earnings calls, tech-press headlines (e.g. 抖音日活突破七亿) and PM / data-analyst chat — not in everyday conversation.