Latin for "things done."
A peculiar rule, used mostly in criminal cases, which allows hearsay if the statement is made during the excitement of the litigated event.
For example, the words "stick ’em up!" used during an armed robbery would be admissible hearsay under the res gestae rule.
So, too, would other spontaneous statements made by the defendant during or right after the crime.
In The Law of Evidence in Canada, Sopinka suggested that res gestae are of 4 varieties:
- declarations accompanying and explaining relevants events;
- spontaneous declarations;
- declarations which by their content create legal rights or liabilities; and
- declarations of "bodily and mental feelings and conditions".
Some laws even allow res gestae statements to be introduced in evidence in special kinds of prosecutions. For example, in child sexual abuse cases, the statement made by a child to another person may be allowed as evidence even though, technically, it offends the rule against hearsay. 
This is to recognize the trauma of having a child testify in open court on the subject of her or his abuse.
Res gestae evidence usually requires a voir dire hearing before it is admissible unless the defense allows it to be put on the trial record unchallenged.