A "landmark decision" is a very important ruling or judgment made by a court or a governing body that sets a new precedent or significantly changes the way certain things are done or understood. Full definition
I was curious and searched for «leading cases» and found an entry for landmark decisions which I found very good. (slaw.ca)
Members have long been known for appearing in landmark decisions in complex discrimination and equality cases, from Seymour - Smith through to more recent cases such as O'Brien v Ministry of Justice and Council of Immigration Judges (where Monckton members appeared on both sides). (monckton.com)
Today marks the 35th anniversary of the U.S. Supreme Court landmark decision on abortion, Roe v. Wade. (slaw.ca)