decepción
/deθebˈθjon/ · B2 · noun · f
disappointment
False friend: decepción is a let-down, not a deception (that's engaño)
Both come from Latin decipere, to trick or ensnare. Spanish kept the sense of being let down by hopes that tricked you, so a decepción is the feeling after. Deception in Spanish is engaño. Verb: decepcionar = to disappoint.