Jealousy is a gameplay feature in The Sims series. Jealousy occurs when a Sim who has romantic feelings for another Sim observes their partner engaging in unfaithful behavior. When this happens, the Jealous Sim becomes angry with the partner, often to the point of fighting, and the jealous Sim's relationship with the partner is severely harmed. Additionally, a jealous Sim will also become angry with the Sim that the partner was romantically engaged with, and the relationship between the jealous Sim and the other Sim is also harmed.
This game feature existed in The Sims and The Sims 2 base game, but was expanded on in The Sims 2: Nightlife with the introduction of the fury system. Jealous Sims will become furious at their partner if they observe cheating, and will become furious at the Sim that their partner was cheating with.
It is noteworthy that a Sim can hold romantic feelings for another Sim without those feelings being reciprocated. Sims will become jealous if their "partner" is unfaithful, even if the partner does not view the jealous Sim romantically. Thus, a Sim can become jealous even if there is no actual romantic relationship between the two Sims.
Additionally, a jealous Sim's partner does not even need to initiate an unfaithful interaction in order to be considered cheating; the partner can be the target of romantic interactions from another Sim, which can cause jealousy even if the unfaithful partner didn't initiate the interaction.