The procedures that must be followed when filing, preparing for trial, and trying civil (meaning non-criminal) lawsuits. Most often, these procedures are compiled in "rules of civil procedure." The federal rules of civil procedure (2023 edition) apply in federal courts. Each state has also adopted its own rules of civil procedure, though many states base their rules on the federal rules.
Civil procedure governs only form and procedure, not the substantive laws that give people the right to sue or defend a lawsuit. Different rules apply to different kinds of cases. For instance, in criminal matters, the "rules of criminal procedure" will apply. When a case is appealed, the "rules of appellate procedure" control.