Reverse Turing test if the subject is or is not human. As of 2004, such procedures, called CAPTCHAs, are used in some anti-spam systems to prevent automated bulk use of communications systems. The use of captchas is controversial. Circumvention methods exist that reduce their effectiveness. Also, many implementations of captchas (particularly ones desired to counter circumvention) are inaccessible to humans with disabilities, and/or are difficult for humans to pass. Note that "CAPTCHA" is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart" so that the original designers of the test regard the test as a Turing test to
Turing completeness is that there exist problems for which it is impossible to predict what a (Turing-complete) system will do over an arbitrarily long time. The classic example is the halting problem: create an algorithm which takes as input (a) a program in some Turing-complete language, and (b) some data to be fed to "that" program; and which determines whether the program, operating on the input, will eventually stop or will continue forever. It is trivial to create an algorithm that can do this for "some" inputs, but impossible to do this in general. For any characteristic of the program's eventual output,