What is the Exclusionary Law?

Answer 1

There is the exclusionary rule (not law) that is a legal principle holding that evidence collected or analyzed in violation of the U.S. Constitution is inadmissible for a criminal prosecution in a court of law.
There are other types of exclusionary rules (the above is the most common usage), but in those other uses, it refers to any action, instrument, or practice that is excluded from being admissible in a court of law, or anything that is excluded from being put into use under the authority of law.
Hope this helps!

Answer 2

It's actually the Exclusionary Rule, it's a rule of evidence. If the police gained any evidence through a violation of the defendant's Constitutional Rights, it can't be used at trial. That's what the Miranda case was all about.

I'm not going into them here, but there are exceptions to the rule.