A caltrop is a small object with four sharp spikes arranged such that however it lands on the ground, three spikes are down and one is pointing up.  Ninjas are said to have tossed these on the ground as they ran away to stop barefoot pursuers.
A caltrop argument is a defensive argument that attempts to avoid an argument rather than respond to it honestly.
My favorite caltrop argument goes something like this:

Atheist: There is no absolute truth beyond trivial statements like 1 + 1 = 2.
Christian: Well, that certainly sounded like an absolute truth statement!  Aha—you’ve defeated yourself!
Atheist: [sigh]  Fine.  What I should have said was “I have never seen evidence of such absolute truth statements.”

The atheist in this exchange made a mistake.  But instead of interpreting the statement charitably and finding the valid point wrapped in an imperfect presentation, the Christian tried to use the mistake to avoid the point completely.
Of course, I’m not saying that only one group is guilty of this.  Atheists can toss out caltrops to avoid confronting an argument as well.  But the person interested in the truth confronts an argument directly.
Related posts:

• See all the definitions in the Cross Examined Glossary.