AB / Multivariate Testing.

A technique for testing a hypothesis in which multiple variables are modified. The goal of multivariate testing is to determine which combination of variations performs the best out of all of the possible combinations.

The A/B Test: Inside the Technology That’s Changing the Rules of BusinessWired
What is Multivariate Testing?Optimizely
The Ultimate Guide To A/B TestingSmashing Magazine
10 Things To Know About A/B TestingMeasuring U
Putting A/B Testing in Its PlaceNielsen Norman GroupAugust 14th, 2005