百亿 (bǎi yì) is a Chinese numeral meaning 'ten billion' (10¹⁰, or 10,000,000,000). It combines 百 (bǎi, 'hundred') and 亿 (yì, 'hundred million'), where 亿 itself equals 10⁸ — so 百亿 literally means 'one hundred × one hundred million', yielding 10² × 10⁸ = 10¹⁰. Unlike English, which uses 'billion' for 10⁹, Chinese counting follows the traditional East Asian system where 亿 = 100,000,000.

This term appears frequently in financial, demographic, and scientific contexts — for instance, reporting national budgets, corporate valuations, or population estimates. It is formal and precise, rarely used in casual speech unless discussing large-scale figures. Learners should note that 百亿 is not interchangeable with 'billion' in English translations without adjusting the numeric base, and it always functions as a quantifier before nouns (e.g., 百亿元, 百亿人口), never standing alone as a noun.