1.
ten million, 10,000,000
The number 10,000,000, written with the explicit 一 (one) prefix on 千万 (ten million) for clarity, most often in formal documents, contracts, and on checks or invoices where the amount must be unambiguous.
一千万 (ten million) is the same value as 千万, but with the 一 (one) written explicitly. Ordinary speech usually drops it and just says 千万えん, but 一 is added in formal writing to prevent the number from being altered or misread.
Usage
Seen especially on legal documents, contracts, receipts, and bank forms, where 一千万 makes clear that no digit precedes 千万. Formal kanji numerals (壱千万, using 壱 instead of 一) are used on checks and official documents for the same anti-fraud reason.