What does common law mean in the first clause of the 7th Amendment?In suits at common law, where the value in controversy shall exceed twenty dollar

The term "common law" is one that is used more by the
British than by Americans, which is one reason why the Framers of the Constitution used
it even though it is not so familiar to us today.  The common law is law that has been
created by the precedents set in court cases.  This is in contrast to law that has been
set out by statutes passed by legislatures.

The basic idea
behind common law is that the decisions made by courts in the past should determine what
the law is today.  This is the idea of stare decisis--the idea that
precedents from past cases should be honored unless there is a very good reason not to. 
This idea is generally applied to torts and contracts, not to criminal
law.

So, common law is law that is based on precedent from
past court cases, not on law.