The adjective authentic describes something that is real or genuine and not counterfeit. Be careful when you are buying jewelry or watches. If you are going to buy a diamond ring, you want an authentic diamond and not glass. That would be authentic glass but a fake diamond!
In addition to describing something real, the adjective authentic describes something reliable, based on fact, and believable. If you have to write a research paper for school, be sure to use authentic facts and figures. The teacher is sure to check if you write, "My little sister told me that the Eiffel Tower is about 10,000 feet high," when in fact it's 1,063 feet tall.