Principal Register

Definition from Nolo’s Plain-English Law Dictionary

The primary list on which trademarks that meet certain federal filing standards are placed. The benefits of getting a mark placed on the Principal Register include the notice to potential copiers that your mark is valid and protected, the right to sue to stop copying, and the right to have the mark considered immune from legal challenge after five years of continuous use.

Definition provided by Nolo’s Plain-English Law Dictionary.