What is "best evidence rule"

n. a rule of evidence requiring that to prove the content of a writing or recording or photograph the original is required

The best evidence rule is a legal principle that holds an original copy of a document as superior evidence. The rule specifies that secondary evidence, such as a copy or facsimile, will be not admissible if an original document exists and can be obtained. The rule has its roots in 18th century, British law.