They are called "unconstitutional laws".
Judicial Branch
The legal concept or theory whereby a court declares a law unconstitutional is commonly called "judicial review." This was not so much established as confirmed by the US Supreme Court case Marbury v. Madison.
Nullification
In the US, when congress eliminates a law this is called a repeal. When the Supreme Court finds a law unconstitutional, this is called "striking down" a law.
The judicial branch can say if a law is unconstitutional.
Any practice or law that violates the constitution is unconstitutional.
A law that violates the constitution is said to be unconstitutional.
The Judicial Branch
No, an Appeals Court cannot 'find' a law unconstitutional. They might declare a law to be unconstitutional IN THEIR BELIEF, but they can only overturn the decision of the lower court and/or return it to them for further action or consideration. Only the U.S. Supreme Court can find a law unconstitutional.
Congress can REPEAL any law, constitutional or not. Only the Court can overturn a law because it is unconstitutional.
Yes, a law can be overridden if it is challenged as unconstitutional. If a court determines that a law violates the constitution, it can be declared invalid and unenforceable.