Yes the semantic translation is "original evidence". That's the meaning. But when you read them in accounting or auditing textbooks, they are called "source documents". Source documents are your sales invoices, receipts, purchase and payment (or cheque) vouchers.

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Note added at 12 hrs 27 mins (2005-05-05 18:43:46 GMT)
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A source document may not be a voucher as such. It may simply be an approved calculation sheet or an auditor\'s worksheet. So I think stonejohn\'s understanding is not exactly what is used in accounting textbooks overseas, although Commuease and jcproz may have agreed with him.