Dictionary.com
definitions

crystal set

noun
  1. an early, tubeless radio receiver using a crystal detector.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for crystal set

crystal set

noun
  1. an early form of radio receiver having a crystal detector to demodulate the radio signals but no amplifier, therefore requiring earphones
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