A body, whether whole or in part, which has been examined by a medical doctor/forensic pathologist or geneticist to name a few possibilities and conclusively determined to NOT be of terrestrial origin. Or material, whether clothing or structural, which can be determined as NOT having originated on earth. This would be evidence for the existance of ET's. Speculation is not evidence of such, nor are anonymous claims from other people, nor redacted documents which can be interpreted any way one chooses. And all of this without the believers convenient claims of "they don't want to be made public" or "it's a cover-up to withhold information". These are no more than attempts to justify having nothing.