Uplift
Uplift

Definition
An uplift is the percentage or value of an increase in the conversion rate or other success metric of the challenger variant compared to the control variant.
In the example below the default conversion rate for registration completion is 10.48%. Variant 2 though achieved a conversion rate for registration of 13.72%. If we divide the variant 2 conversion rate (13.72%) by the default (10.48%) and multiply by 100 we get 130.91% or an uplift of 30.91%.

Source: 2016 Conversion-uplift
Resources:
Conversion marketing – Glossary of Conversion Marketing.
Over 300 tools reviewed – Digital Marketing Toolbox.
A/B testing software – Which A/B testing tools should you choose?
Types of A/B tests – How to optimise your website’s performance using A/B testing.