A witness is a person who is present at the signing of a legal document. They sign the document to indicate that they personally saw the signing and the document is authentic. The date that the witness signs any document should be the same as the date that the tenant/guarantor signs the document. A witness can be anyone over the age of 18. If you are booking a property in England this should not be a family member. The witness must be a third party that is not the tenant or guarantor and should be over 18 years of age.