State's evidence

A popular term for testimony given by an accomplice or joint participant in the commission of a crime tending to criminate or convict the others, and given under an actual or implied promise of immunity for himself.

— Substantive evidence. that adduced for the purpose of proving a fact in issue, as opposed to evidence given for the purpose of discrediting a witness, (i. e., showing that he is unworthy of belief.) or or corroborating his testimony. Best, Ev. 246, 773, 803.
—-Substitutionary evidence. Such as is admitted as a substifute for what would be the original or primary instrument of evidence; as where a witness is permitted to testify to the contents of a lost document.

Defined under Evidence in Black's Law Dictionary.