Prior and Prelusive

Prior adjective - Coming before all others in importance.

Prelusive and prior are semantically related In some cases you can use "Prelusive" instead an adjective "Prior".

Prelusive adjective - Coming before the main part or item usually to introduce or prepare for what follows.

Prior and prelusive are semantically related in preliminary topic. You can use "Prior" instead an adjective "Prelusive", if it concerns topics such as introductory.