A cause of action, in law, is a set of facts sufficient to justify a right to sue to obtain money, property or the enforcement of a right against another party. It also refers to the legal theory upon which a plaintiff brings suits such as breach of contract, battery or false imprisonment. There a number of specific causes of action, including contract-based actions, statutory cases of action, torts such as assault, battery, invasion of privacy, fraud, slander, negligence and suits in equity such as unjust enrichment and quantum meruit.
In order to pursue a cause of action, a plaintiff pleads or alleges facts in a complaint, the pleading that initiates a lawsuit. A cause of action covers both, the legal theory and the remedy. The defendant to a cause of action must file an “answer” to the complaint in which claims can be admitted or denied.
The term cause of action is mentioned in the Civil Procedure Code, 1908. Order I Rule 8 explains that the parties being represented in the suit need not have the same cause of action as the person they are being represented by. Order II Rule 2 of the code states that a plaintiff must mention the cause of action if it is to be instituted as a suit. To win a case the plaintiff must prove the major legal points of the case lie in his favour and it is called the elements of that cause of action.

There are three important factors which cause of action decides in each and every civil suit. They are:
1. Jurisdiction
2. Limitation
3. Rejection of plaintiff

Cause of action arises at any following places:
1. Place of contract
2. Place where contract was to be performed or performance
3. Place where money was expressly or impliedly payable

The cause of action is often stated in the form of a syllogism, a form of deductive reasoning that begins with a major premise (rule of law), proceeds to a minor premise and ends with a conclusion. The facts or circumstances that entitle a person seek judicial relief may create more than one cause of action. A cause of action can arise from an act, a failure to perform a legal obligation, a breach of duty or a violation or invasion of a right. A set of facts may have no legal effect in one situation, whereas the same or similar facts may have significant legal implications in other situations.

T. Muralidhar V. PVR Murthy
(RFA (OS) No. 115/2014 & CM No. 12344/2014)
Judge Gita Mittal held that what is not pleaded, cannot be proved, what is admitted need not be proved. It is this fundamental principle which binds every court. It is very principle which shall be violated if the present appeal is accepted.
Cause of action is cause of action for which the suit was brought. Cause of action is cause of action which gives occasion for and forms the foundation of the suit. If that cause of action enables a person to ask for a large and wider relief than that to which he limits his claim, he cannot afterwards seek to recover the balance by independent proceedings.