It depends on the context, but modern use of the term generally applies to the question of whether evidence seized in violation of the Fourth Amendment can be used against a person in criminal proceedings. The Republic's founding generation understood that things seized in violation of the Constitution could not come into evidence in court, because lawlessness by government has no place in the hallowed halls of American courts. Judges now are not as concerned about the purity of what comes into court as they are about helping their police friends "do justice," in a result oriented way. So the fiction was created, by the Nixon justices in the 70's, that there is some "rule" of exclusion that is different and apart from the Constitution itself, and the question increasingly is asked now whether evidence unconstitutionally seized should be "excluded" from court, and under what circumstances, so that "exclusionary rule," loosed somewhat from the moorings of the Constitution, is a shifting concept, depending on the politics of the courts that are employing it. Evidence seized in violation of the 5th and 6th Amendment is also not admitted in court, but that is generally deemed to result merely from an "objection" and not an exclusionary rule.