Chlorogenic acid (CGA) is an antioxidant and phenolic compound found in the skin, leaves and fruits of a number of plants, including the "fruits" of the coffea arabica plant - unroasted ("green") coffee beans. It is believed to be the substance which makes green coffee extract an effective supplement for weight loss and for controlling hypertension.
Chlorogenic acid molecules are made up of caffeic acid and L-quinic acid. It got its name because it takes on a green color, like chlorine gas, when oxidized.
Raw coffee beans are one of the most abundant sources of this compound, however much of it is destroyed when the beans are roasted.
Other foods rich in chlorogenic acids include:
and in lesser amounts:
(Chlorogenic acids are believed to be responsible for the notorious "laxative effect" caused by eating prunes.)
Additional information and studies on chlorogenic acid and green coffee can be found at PubMed - the online site of the US National Library of Medicine.