Meaning of seniority rule

senior'ity rule'

Pronunciation: [key]
— U.S. Pol. U.S. Pol.
  1. the custom in Congress providing for the assignment of a committee chairpersonship to that member of the majority party who has served on the committee the longest.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.