Ammonia is considered a compound because it is made up of two or more different elements chemically bonded together. Specifically, ammonia (NH3) consists of one nitrogen atom and three hydrogen atoms. These atoms are held together by covalent bonds, which means they share electrons to achieve stability.
In the case of ammonia, the nitrogen atom shares its electrons with the hydrogen atoms, forming a stable molecule. This combination of different elements in a fixed ratio and the presence of chemical bonds are key characteristics that define a compound. Therefore, ammonia fits the definition of a compound perfectly.