prime ministern. Abbr. PM1. A chief minister appointed by a ruler.2. The head of the cabinet and often also the chief executive of a parliamentary democracy.prime ministerial adj.prime ministership, prime ministry n.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.