That part of a chimney directly above the fireplace where the walls of the flue are brought close together as a means of increasing the draft.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved
chimney throat, chimney waist
The narrowest portion of a chimney flue, between the gathering (or upward contraction above the fireplace) and the flue proper; often where the damper is located.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.