What is the technical term for the code to stop people from automatically siging up for things?

Question

The box with random numbers and letters with various colors and sometimes gridded, used as a security feature on some sites.

Answer 1

captcha

"
A captcha (an acronym for "completely automated public Turing test to tell computers and humans apart") is a type of challenge-response test used in computing to determine whether or not the user is human. The term was coined in 2000 by Luis von Ahn, Manuel Blum, and Nicholas J. Hopper of Carnegie Mellon University, and John Langford of IBM. ...
http://en.wikipedia.org/wiki/Captcha
"