Freethought is a philosophical doctrine that says beliefs should be formed by scientific, empirical and logical enquiry and is not to be compromised by dogamtic or else fallacious beliefs that restrict logical reasonng. Freethought is sometimes seen as synonymous with atheism or agnosticism. Freethought is a system of belief based on logic and the Scientific method and which seeks to avoid ideas based on simple faith.
Freethought can also apply to fields outside religion like politics, consumer choice and analysis of paranormal claims. indeed freethought is not actually the same as atheism. One might imagine that freethinkers could, at least theoretically, find scientific evidence for religion. Despite this as evidence which supports religious beliefs is non-existent, freethinkers are almost always atheists or agnostics.
Those who practise freethought are called, naturally enough, freethinkers.