What would you consider an unfit parent?

We’re having a lot of problems with our child’s bio parent. CPS has went to the house multiple times because the child comes home in horrible shape, CPS says every time that they seen some concerning things but they always close the case after a few months. It’s so easy to lie and manipulate CPS.... I just want to know what you all think and unfit parent is. I don’t want to get into details but I will say it’s bad and it’s been physical on many occasions. Try not to focus on anything but the actual question lol