A household is one group of playable Sims living on the same lot. In The Sims 2: University, the term was used for young adults living in college sub-neighborhoods, to differentiate them from families living in other areas. The term became more common in The Sims 3, probably because Create a Sim in The Sims 3 lets players specify each Sim's last name. "Family", which was the more common term in earlier games, is still used for households in which all Sims are related.
In all games, Sims can be added to a household by birth, adoption, or marriage/moving in, and can be removed by death or marrying into/moving into another household. Beginning with The Sims 2, Sims could move out on their own. Beginning with University, players were able to merge two households. This feature was available in all expansion packs for The Sims 2, regardless of whether University was installed or not. This provided an indirect way to create non-college households in which Sims had different last names.
In all games, the number of Sims (and pets, if available) that can be in a household is limited. Without a pet-related expansion pack, household size is limited to eight. In The Sims, the Unleashed expansion pack does not change this limit; a household can have any number of Sims and pets as long as the total does not exceed eight. The pets expansions for The Sims 2 and The Sims 3 keep the eight-Sim limit and limit the number of pets to six, but increase the household size limit to ten. This allows an eight-Sim household to have one or two pets. The Sims 2 Pets for console also increases the household size limit, but limits the number of pets to four.