The word carat comes from the Carob Mediterranean tree whose seed was used for centuries as the standard of weighing precious gems. Each carob seed weighs approximately one-fifth of a gram. A carat is one-fifth of a gram.
Jewelry & Diamond Industry Dictionary of Terminology
GIA is the common abbreviation for the Gemological Institute of America. GIA is the organization which created standard terminology for the diamond industry, and is still considered the authority on the science of diamonds and gemology.
Sources for these definitions are GIA.edu, Google.com, Meriam-Webster.com, and Wikipedia.org.