diff --git a/activesupport/lib/active_support/callbacks.rb b/activesupport/lib/active_support/callbacks.rb
index 4950d009d1..5e0c5b6e98 100644
--- a/activesupport/lib/active_support/callbacks.rb
+++ b/activesupport/lib/active_support/callbacks.rb
@@ -523,7 +523,8 @@ module ActiveSupport
       # This macro accepts the following options:
       #
       # * <tt>:terminator</tt> - Indicates when a before filter is considered
-      # to be halted.
+      # to halted. This is a string to be eval'ed and has the result of the
+      # very filter available in the <tt>result</tt> variable:
       #
       #   define_callbacks :validate, :terminator => "result == false"
       #