Action 指的是具体的某次诉讼。可用于指具体的某一类别的诉讼：如下文的libel action诽谤诉讼、tort action侵权诉讼、civil action民事诉讼、criminal action刑事诉讼、经济诉讼economic action、行政诉讼administrative action，或者与诉讼相关的动作，如起诉begin action 或者bring an action。其多用于描述某一情况。民事诉讼和刑事诉讼皆可使用。

如以下为Cambridge English对其的释义及举例：

a legal process that is decided in a law court

诉讼；起诉

a libel action

诽谤诉讼

She brought an action (for negligence) against the hospital.

她起诉医院（玩忽职守）。

A criminal action was brought against him.

他被提起刑事诉讼。

The book was halted in South Africa by a threat of legal action.

因为遭到起诉的威胁，该书在南非停止发行。

Lawsuit 与Action一样，都是指的具体的某次诉讼，但lawsuit多用于民事诉讼。

The term "lawsuit" is used in reference to a civil action brought in a court of law in which a plaintiff, a party who claims to have incurred loss as a result of a defendant's actions, demands a legal or equitable remedy. The defendant is required to respond to the plaintiff's complaint. If the plaintiff is successful, judgment is in the plaintiff's favor, and a variety of court orders may be issued to enforce a right, award damages, or impose a temporary or permanent injunction to prevent an act or compel an act. （From Wikipedia）

Two of the directors filed a lawsuit against their former employer.

其中的两位董事已经对其前僱主提出诉讼。

mainly UK They brought a lawsuit against the company.

他们对这家公司提起诉讼。

至于litigation，则指的是诉讼这一行为，其是一个抽象名词，并不能指具体某一次的诉讼，比如说，提起诉讼就不能用file a litigation。Actions和lawsuits都属于litigation类别。它们之间的区别就如letter和correspondence之间的区别一样。所有的letter都是correspondence，所有的actions和lawsuits都是litigation。Litigation既可以指刑事诉讼，也可以指民事诉讼。

the process of taking a case to a court of law so that a judgment can be made

诉讼，起诉

The company has consistently denied responsibility, but it agreed to the settlement to avoid the expense of lengthy litigation.

公司一直在推卸责任，但同意这一调解方案以避免漫长的诉讼所带来的花费。

由此可见，提及“诉讼制度”时，讲的是一种抽象的概念，因此使用litigation一词最为合适。下文介绍美国的诉讼制度时，即选用的此词：

The American System of Litigation

Perhaps no other country in the world has developed such a complex, expensive, all pervasive and remarkably effective and influential system of Courts as the United States. It is in the court room rather than the legislature that the United States creates and enforces the bulk of the laws and procedures which most directly effect the business climate and practices within the United States.

Whether in the Federal Courts, which are subject to the federal law created by the United States federal government in Washington DC, or in the individual State Courts, operated within the particular systems of law passed by each State, every individual and corporation, whether domestic or foreign, has the right to file legal action against any individual or entity accused of breaching a contract, violating a duty, or breaking some law, including against the government, itself. Unlike much of the world, the United States has vested tremendous power in the Courts, often power sufficient to countermand the wishes of the Congress or of the President. Judges are often appointed for life precisely to give them independence from the influence of the other branches of the government. Each law passed by Congress or the various State legislatures must pass the test of being in conformity with the Constitution of the United States and that test is applied solely by the Courts.