Short answer. Section 2 of Rule 2 defines it precisely: a cause of action is 'the act or omission by which a party violates a right of another.' You need both elements — a right belonging to you, and a specific act or failure to act by the other party that violated it.
What the law says
A cause of action is the act or omission by which a party violates a right of another.
Rule 2, Section 2 — Cause of action, defined. Read the full provision →
The two-part definition
Section 2's definition is compact but contains two necessary components. First, there must be a right — something the law recognizes as belonging to the plaintiff. Second, there must be an act or omission by the defendant that violates that right. Neither element alone is enough: a right that was never violated gives no cause of action, and a wrongful act that violated no right of the plaintiff's likewise gives none. The violation is what converts an abstract legal right into an actionable claim.
Acts and omissions are both covered
The definition deliberately covers both action and inaction. A cause of action can arise from something the defendant affirmatively did — trespassing on land, breaching a contract term — or from something the defendant failed to do when obligated to act, such as not paying a debt that fell due or not delivering something promised. The Rules do not privilege one form of violation over the other; either can supply the basis for an ordinary civil action under Rule 2.
Why the definition matters at the pleading stage
Because Rule 2's companion section makes a cause of action mandatory for any ordinary civil action, this definition is what a complaint's allegations are ultimately tested against. A plaintiff drafting a complaint needs to allege facts showing both halves of the definition — a right, and a specific violating act or omission by the defendant — clearly enough for a court to see that the two-part definition in Section 2 is actually satisfied.
How this differs from a right of action
A cause of action, as defined here, is distinct from the procedural right to actually bring the suit — a plaintiff can have a genuine cause of action in this substantive sense while still facing a procedural bar, such as an expired prescriptive period, that prevents them from successfully pursuing it in court. Section 2's definition addresses only whether the underlying facts show a right and a violation of it, not whether every procedural condition for suing on that violation has also been met. A defendant faced with a complaint that fails to allege both elements clearly is not without recourse; the Rules allow a motion challenging a pleading that fails to state a cause of action, which tests exactly whether the complaint's own allegations, taken as true, actually satisfy Section 2's two-part definition.