Dictionary.com
definitions

trim tab

or trimming tab

noun Aeronautics.
  1. an independently controlled tab set in the trailing edge of a control surface, as an elevator, aileron, or rudder, to hold it in a position suitable for stabilizing the aircraft in a flight attitude.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for trim tab

trim tab

noun
  1. a small control surface attached to the trailing edge of a main control surface to enable the pilot to trim an aircraft
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