Domestic violence or domestic abuse is when one adult is violent or aggressive towards another adult in the same family, or someone they're going out with, or have been out with.
It doesn't just have to be physically hurting someone; it can be verbally bullying them, controlling what they spend their money on or deciding who they speak to and which friends they go out with.
Around 750,000 children a year are affected by domestic violence. It's thought that one in four women and one in six men will experience some form of domestic abuse in their lives.
It's hard to know exactly how many people are affected because lots of people don't report it to the police.
The law says that any incident of threatening behaviour, violence or abuse between adults who are family members or have been in a relationship counts as domestic violence.
Lots of families argue and have disagreements from time to time, it can be normal healthy part of family life.
If you are worried you should talk to someone you trust, either another adult in your family, a teacher or a friend.