hearsay rule

The rule prohibiting hearsay (out of court statements offered as proof of that statement) from being admitted as evidence because of the inability of the other party to cross-examine the maker of the statement.  See:  hearsay

There are a number of exceptions to the hearsay rule (including present-sense impression, excited utterances, declarations of present state of mind, dyng  and the business records exeptions), as well as things defined not to be hearsay (admission of a party-opponent, and prior statements of a witness).