A hormone is a chemical that acts as a messenger transmitting a signal from one cell to another. Hormones can be classified according to chemical composition, solubility properties, location of receptors, and the nature of the signal used to mediate hormonal action within the cell.

• Cholecystokinin is a hormone that is released after a meal. It plays a key role in facilitating digestion within the small intestine. 
• It seems to be involved with appetite by increasing the sensation of fullness in the short-term, that is, during a meal rather than between meals.
• Cholecystokinin stimulates the release of bile juices from the liver and gall bladder which improves digestion.
• It regulates the movement of food through the stomach that helps in the effective absorption of nutrients from the food.
• During this process, the human body feels satiated/full and does not feel hunger. Thus, the release of cholecystokinin hormone decreases the appetite.

Thus, it is concluded that Cholecystokinin is a hormone which when injected into the human body decreases appetite.