Voucher is the certificate issued by Travel service company that CAN be exchanged for Travel related Expenses such as lodging, air tickets, sightseeing, meals and more.
Other definition:
Voucher is the document to be exchanged for goods or services, substantiating that payment has already been made. Also called an exchange order.
Other definition:
Voucher is the documents that are exchanged for goods and service to substantiate payment that will be or already has been made
The Travel-Glossary: Everything about Travelling, Sightseeing and the World.