Southeast Asia A region of Asia bounded roughly by the Indian subcontinent on the west, China on the north, and the Pacific Ocean on the east. It includes Indochina, the Malay Peninsula, the Malay Archipelago, and sometimes the Philippines.Southeast Asian adj. & 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.Southeast Asia n (Placename) a region including Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Thailand, and Vietnam Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014South?east A?sia n. a region including Indochina, the Malay Peninsula, and the Malay Archipelago. South?east A?sian, n., adj. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.