A statute of limitations sets a maximum time after an injury or crime has occurred to bring a related suit or charges. When a statute of limitations is suspended, it is said to be "tolled." Equitable tolling is a legal doctrine allowing for a delay or suspension of the running of time set forth in a statute of limitations.
A statute of limitations is meant to protect a defendant. Tolling provisions are meant to aid a plaintiff by extending the time within which he is permitted to bring suit. Equitable tolling stands for the same principle; it is a legal doctrine found predominantly in tort law and civil procedure allowing for a suspension of a statute of limitations.
The term "equitable" is derived from a set of legal principles espoused in equity law. Equity law stands for that which is fair and just; it blunts strict rules of law where their application could cause an unduly harsh result. Thus, in tort law, equitable tolling simply means that a statute of limitations will not bar a claim if a plaintiff — in due diligence — did not or could not discover the injury in question until after the expiration of the statute of limitations. The best example of this principle is in a medical malpractice context. If a surgeon leaves a surgical sponge inside a patient but the patient doesn't discover the injury until after the statute of limitations has run out, the patient will be allowed to file a medical malpractice claim based on principles of equity; disallowing the claim would be unjust.
In civil or class-action lawsuits, equitable tolling is typically applied through the rules of civil procedure. Civil procedure is a body of law pertaining to the rules that courts follow when adjudicating civil suits. The rules of civil procedure govern how a lawsuit may be filed, the timing and/or manner of depositions, motions and pleadings. Once a civil suit or class-action suit has been filed, the statute of limitations is tolled.
The doctrine of equitable tolling generally applies in the context of a civil suit in which a defendant has moved to dismiss the suit based on a plaintiff's inability to comply with the rules of civil procedure or has failed to comply with the statute of limitations for the claim in question. For example, a defendant may move to dismiss a case by arguing that the plaintiff ran afoul of a statutory time limit on filing the suit, filing a motion or adding an indispensable party. In such an instance, a court may apply the legal remedy of equitable tolling if the plaintiff can show good cause for exceeding the statute of limitations. Examples of good cause for exceeding a statute of limitations could include the inability to discover an injury or indispensable party to the suit within the statutory period, or an earlier filing of an identical civil action in a different jurisdiction.