A more thorough explanation:

First impression is a legal term used to describe a situation where a court is faced with a new issue or interpretation that has not been addressed before. This means that there is no existing legal precedent or authority on the matter.

For example, if a court is presented with a case that involves a new law that has just been passed, it may be a case of first impression. Similarly, if a court is presented with a case that involves an issue that has not been addressed within its jurisdiction, it may also be a case of first impression.

When a court is faced with a case of first impression, it may seek guidance from other jurisdictions or make analogies to related or similar issues. This is because there is no existing legal authority to rely on.

For instance, in the case of Fiore v. White, 562 Pa. 634, the court held that a case of first impression is one that presents an "entirely novel question of law" that "cannot be governed by any existing precedent". This means that the court had to rely on its own judgment and reasoning to come up with a decision.

Overall, a case of first impression is an important legal concept that highlights the need for courts to carefully consider new issues and interpretations that have not been addressed before.