Dictionary.net

A specific legal claim -- such as for negligence, breach of contract or medical malpractice -- for which a plaintiff seeks compensation. Each cause of action is divided into discrete elements, all of which must be proved to present a winning case.

By this phrase is understood the right to bring an action, which implies, that there is some person in existence who can assert, and also a person who can lawfully be sued; for example, where the payee of a bill was dead at the time when it fell due, it was held the cause of action did not accrue, and consequently the statute of limitations did not begin to run until letters of administration had been obtained by some one. 4 Bing. 686.

There is no cause of action till the claimant can legally sue, therefore the statute of limitations does not run from the making of a promise, if it were to perform something at a future time, but only from the expiration of that time, though, when the obligor promises to pay on demand, or generally, without specifying day, he may be sued immediately, and then the cause of action has accrued. 5 Bar. & Cr. 860; 8 Dowl. & R. 346.When a wrong has been committed, or a breach of duty has occurred, the cause of action has accrued, though the claimant may be ignorant of it. 3 Barn. & Ald. 288, 626 5 B. & C. 259; 4 C. & P. 127.