arena theatern. A theater without a proscenium, in which the stage is at the center of the auditorium and is surrounded by seats. Also called theater-in-the-round.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.are?na the?ater n. a theater with seats arranged on at least three sides around a central stage; theater-in-the-round. [1940?45] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.