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
The chemical element of atomic number 6, a nonmetal which has two main forms (diamond and graphite) and which also occurs in impure form in charcoal, soot, and coal. People outside of the jewelry industry often call imperfections within diamonds, "carbon," or "carbon spots." However, within the jewelry and diamond industries these imperfections are called, "inclusions," or "blemishes."
Sources for these definitions are GIA.edu, Google.com, Meriam-Webster.com, and Wikipedia.org.