Moodlet
Moodlets are mood altering events or situations that a Sim has done or experienced. These moodlets may be positive, negative, or neutral. Some moodlets happen for an indefinite amount of time while others are timed. Some moodlets lead directly from one to the next, such as "Strained" leading to "Stressed Out", but all other moodlets are not mutually exclusive and so you can have "Vile Surroundings" and "Beautifully Decorated" at the same time, or multiple instances of "It's a Boy!".
Moodlets that do not specify a time only end when certain events occur, such as leaving the area that gives the moodlet.
Moodlets may also have secondary effects, such as improving skill gain, making it easier to burn to death, or making a sim fatter when they eat, to name a few.
Moodlets can be removed using the Moodlet Manager lifetime happiness reward. The manager is not perfect, and may affect the sim with a red beam causing negative effects instead of removing them.