What is a tort?
A class action lawsuit is called a tort, but I don't know if that's what you mean.
A tort, in common law jurisdictions, is a civil wrong which a person causes someone else to suffer loss or harm resulting in legal liability for the person who commits the tortious act.
B3AST is correct - a tort is a civil wrong. Basically, things like negligence (99% of all auto collision cases) and things that you sue people for, other than breach of contract cases.