* Απογοητευμένος (apogoitefmenos): This is probably the closest single word translation. It implies a feeling of letdown, frustration, and loss of hope.
* Απογοητευμένος (apogoitefmenos) can also be used to translate "disheartened".
Other options might require more context:
* Words expressing sadness or unhappiness could be used depending on the situation, such as λυπημένος (lipimenos) (sad) or θλιμμένος (thlimmenos) (sorrowful).
* If the disappointment stems from someone failing to meet expectations, words related to betrayal or frustration might be more suitable.
Therefore, απογοητευμένος (apogoitefmenos) is the most common and accurate single-word translation, but the best choice will depend on the specific context of the disappointment.