1. Discouragement, disappointment; a slackening of effort; relaxation
2. The descent of an aircraft or spacecraft to the point at which a landing approach is begun
3. A physiological response of a lactating mammal to suckling and allied stimuli whereby previously secreted milk from the acini is expelled into ducts and drawn through the nipple
4. When you say that something is a letdown, you are disappointed because you cannot have or cannot do something you want to or because something is less exciting than you expected it to be.