(noun)
broadly, the action of not meeting someone's expectations or hopes, or an instance of this, often causing them to feel displeased and/or upset;
e.g. He added to the disappointment of their hopes by refusing to join them.
(noun)
a person, thing, or situation/event that fails to meet one's hopes and expectations and that thus makes one feel displeased or upset (i.e. makes one feel disappointed);
e.g. The concert was a disappointment because the performers were not well-rehearsed.