The current version of the Codex says this of "Defrost":
'this is a trait of flame attacks. It will be defined in a future expansion of the game in which frost damage is introduced.'
We now have Frost damage, but the Elementalist's little rulebook says nothing about Defrost.
Burn tokens and Freeze tokens cancel each other out, but flame attacks do not necessarily cause a Burn, so that can't be it.
Have I missed a memo somewhere that explains Defrost?