Definition:
Split testing (also referred to as A/B testing or multivariate testing) is a method of conducting controlled, randomized experiments with the goal of improving a website metric, such as clicks, form completions, or purchases. Incoming traffic to the website is distributed between the original (control) and the different variations without any of the visitors knowing that they are part of an experiment. The tester waits for a statistically significant difference in behavior to emerge. The results from each variation are compared to determine which version showed the greatest improvement.
Further Reading:
Book: The Lean Startup by Eric Ries