You might be interested in this thread on the topic.

The term "Caucasian" for "White people" was coined by Christoph Meiners in his book "The Outline of History of Mankind" in 1785.

The term in itself had no scientific credibility on its own and was spread,and given scientific foundation, by Johann Friedrich Blumenbach (1752-1840).

Blumenbach studied at first the different facial features, skin colours and cranial measurement. He made out of his studies his own racial groups, expanding on the popular theory by Meiners. The racial groups were as following:

• Caucasian, for the "white race"
• Mongoloid, for the "yellow race"
• Malayan, for the "brown race"
• Ehtiopian, for the "black race"
• American, for the "red race"

He would later change this after he noticed that some people in Europe, in the East and around the Mediterranean sea, were slightly darker in skin colour with an "olive-tint". He would then use his studies of bones and skulls to consider the darker skinned Europeans to still fit into the Caucasian race.

Most studies of racial groups were then based on this work, mostly accepting these rather arbitrary measurements for races.

It is not tied much to the actual Caucasian peoples of Caucasus other than the Georgians, which Blumenbach based heavily his classification on after studying a Georgian Skull.

These theories are today, especially after WWII, discarded by European Nations. And the word Caucasian in Europe is no longer a synonym for "white". But it is still in use in America on an official basis of medicine and ethnical classifications.

I am not sure how credible you or this subreddit finds Wikipedia as source, but, the sources are:

• Caucasian Race
• Johann Frierich Blumenbach
• Scientific racism