Admissible evidence is evidence that is allowed by the trial judge to be used in court.  This is evidence that can be presented to the judge and/or jury that is considering the case.  The opposite of admissible is inadmissible; this is when when evidence is not allowed to be presented in court.  Different jurisdictions may have differing rules on what evidence is considered admissible for a particular case.  With that being said though, many of the US jurisdictions have very similar rules on whether a particular piece of evidence is deemed admissible in a court of law.  It should also be noted that federal and military courts also have their own rules on admissible evidence.  Most states generally will follow the federal rules of evidence.

Example

In a Guam Criminal Defense case,  the lawyer for the prosecution submits a testimony from a bystander.  However, the bystander was not able to make it to the court for the trial.  The trial judge decides that under the hearsay rule, testimony from a witness not in the court would not be admissible evidence in the trial.  Later in the week, the prosecution announces that the bystander is now able make it to the trial to provide firsthand testimony.  Under the new situation, the trial judge deems the testimony admissible evidence.