Thing·vel·lir (th?ng?v?t?lîr?) A broad valley in southwest Iceland east of Reykjavik, noted for its rocky chasms formed by the spreading of the North American and Eurasian tectonic plates. It was the meeting place of the Althing from 930 until 1798. 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.