A more thorough explanation:

Definition: A test action is a legal term that refers to a lawsuit brought to establish an important legal principle or right. It is frequently brought by the parties' mutual consent on agreed facts.

For example, if there is a dispute between two parties about a legal issue that has not been decided before, they may agree to conduct a test case. This means that they will not use unnecessary forms or technicalities, and will mutually admit facts which they know to be true, and without requiring proof. The point in dispute will be brought before the court for decision, without subjecting each other to unnecessary expense or delay.

Another example of a test action is when several suits are based on the same facts and evidence, raise the same question of law, and have a common plaintiff or a common defendant. In this case, the court may select one action as a test case, and all parties are bound by the decision in that case.

Overall, a test action is a way to establish important legal principles or rights in a cost-effective and efficient manner.