let·down or let-down 
(lĕtdoun′)
(lĕtdoun′)n.
1. A decrease, decline, or relaxation, as of effort or energy.
2. A disappointment: The cancellation of the game was a real letdown.
3. The descent made by an aircraft in order to land.
4. A physiological response in lactating females, activated usually in response to sucking or crying by an infant, in which milk previously secreted into the alveoli of the breasts is released into the ducts that lead to the nipple.