What is the accounting definitions of voucher?

You will almost never see this term used in an accounting text book. But a Journal Voucher (JV) or General Voucher (GV) or Voucher all refer to the same thing. (Each company has their own language / terms. ) It is a document used to record basic data that will be posted to the accounting system. It is used as a part of a company's internal control system. It will document accounts to debit / credit, amounts, currency, and prepared by and approved by, etc.

The underlying purpose of a JV is to create an audit trail, so that weeks, months, or years later if a question arrises regarding a transaction there is documented evidence and support for why a transaction was posted or posted in the manner that it was posted.