Definition:
Fitts’ Law is a model of human movement which is a cornerstone of user interface research. The law predicts that the time taken to quickly to a target area is a function of the ratio between the distance to the target and the width of the target.

Resources:
Conversion marketing – Glossary of Conversion Marketing.
Over 300 tools reviewed – Digital Marketing Toolbox.
A/B testing software – Which A/B testing tool should you choose?