from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. someone who belongs to a group that secretly undermines another organization
from the GNU version of the Collaborative International Dictionary of English
- n. see fifth columnist in the vocabulary.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a member of a clandestine subversive organization who tries to help a potential invader
From fifth column + -ist (Wiktionary)