Pathological jealousy is when someone feels really, really jealous and it starts to affect their life in a bad way.
Imagine you have a favorite toy that you love playing with. But then one day, your friend comes over and starts playing with that toy too. You might feel a little bit jealous, but that's okay because you are still happy to have your friend around.
But imagine if you got so jealous that you started to feel very angry and upset, and you didn't want to share your toy with anyone. You might even start to think that your friend is trying to take your toy away from you, even if that's not true.
Pathological jealousy is like that, but with people instead of toys. Someone might feel very jealous if they think their partner is talking to someone else, or going out with their friends without them. They might get very angry and upset, and start accusing their partner of doing things that aren't really true.
This kind of jealousy can be dangerous because it can lead to problems in relationships and can even cause someone to hurt themselves or others. It's important for someone who feels this way to get help from a therapist or counselor, so they can learn how to manage their jealousy and have healthy relationships with the people they love.