One that is immune from criticism, often unreasonably so: "The need for widespread secrecy has become a sacred cow” ( Bulletin of the Atomic Scientists). (noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.