That which, according to established legal principles, cannot be received into evidence at a trial for consideration by the jury or judge in reaching a determination of the action.
Evidence, for example, that is obtained as a result of an unlawful Search and Seizure is inadmissible, as is Hearsay.
INADMISSIBLE. What cannot be received. Parol evidence, for example, is inadmissible to contradict a written agreement.