Noun

1. a hollow wooden horse by which the Greeks gained access to Ilium or Troy.
2. a subversive person or device placed within the ranks of the enemy
3. a malicious program that is disguised as legitimate software
   • 1991. Katie Hafner & John Markoff. Cyberpunk: Outlaws and Hackers on the Computer Frontier (1995 revised edition), Simon and Schuster ISBN 0684818620 p. 255-256

     Worse than what could be observed about the program was the fear that it might be a Trojan horse program -- apparently innocent, but carrying a string of code instructing the computer to carry out a specific damaging instruction at some later time.

4. an offer made to lure customers by seeming like a good deal, but has the ultimate effect of extorting large amounts of money from the customer
5. a social movement, political bill, or ideology with a negative agenda or evil intentions under the guise of positive values or good intentions
   • 2003. Krugman, Paul R. The Great Unraveling: Losing Our Way in the New Century (2004 reprint), W. W. Norton & Company ISBN 0393326055 p. 449

     Indeed, it may turn out to be a Trojan horse that finally allows conservative ideologues, who have unsuccessfully laid siege to Medicare since the days of Barry Goldwater, to breach its political defenses.

Quotations

• 1981, Michael R. Hill "Positivism: A 'Hidden' Philosophy in Geography" in Themes in Geographic Thought edited by Milton E. Harvey & Brian P. Holly; Taylor & Francis ISBN 0709901887 p. 48-49

  Thus positivist ideas can slip into geography just as the Greeks slipped into Troy via the belly of a wooden horse. .... Fortunately, the Trojan Horse hypothesis today describes fewer and fewer geographers.