A more thorough explanation:

Definition: Primae impressionis (prI-mee im-pres[h]-ee-oh-nis) is a Latin term used in law to refer to the first impression. It is often used in the context of a case of first impression, which is a legal case that presents a new issue of law that has not been decided before by any controlling legal authority in that jurisdiction.

Examples:

• A case of first impression is when a court is presented with a legal issue that has not been decided before in that jurisdiction. For example, if a new technology emerges and there is no existing law that addresses it, a court may have to decide how to apply existing laws to the new technology.
• A test case is a lawsuit brought to establish an important legal principle or right. For example, a group of individuals may bring a test case to challenge a law that they believe is unconstitutional.

These examples illustrate how the term primae impressionis is used in the legal context to refer to cases that present new legal issues or seek to establish important legal principles.