an agency employed by advertisers to plan, design, place, and supervise their advertisements or advertising campaigns.
Origin of advertising agency
An Americanism dating back to 1840–50
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for advertising agency
an organization that creates advertising material, contracts for publication space, and sometimes undertakes market research on behalf of its clients
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012