If a person can get you to doubt your judgments then they can get you to instead trust their judgment instead. It is a form of isolating and controlling.
Psychological abuse denies people's realities.
A popular term is gas-lighting based on the old Hitchcock movie where the man refuses to acknowledge his wife's correct assessment that his gas tank is almost empty as indicated by the gas tank indicator on the dash. Ultimately the man convinces the wife that the tank is actually full.
Everyone is sometimes psychologically abusive, but I have seen severe cases where the once confident, independent, and vibrant wife was now so untrusting of her own judgment that she could not even fill out a check without wondering if she was doing it wrong and breaking down into tears. Her friends and family thought she was crazy for leaving her husband because he appeared so confident and caring of her. She had deteriorated under the constant scrutiny and questioning of her actions. When she would try to confront him on his behavior he would deny it or minimize it or declare her crazy and exaggerating. Eventually she started to believe she was crazy.
A metaphor to this would be going on a hike and having the person walking behind you regularly sticking tiny pebbles in your backpack. Over time the pack becomes full of rocks and you get angry, but when you confront the person they say, "It's just a few pebbles, get over it."
Psychological abuse is much more predictive of poor recovery in abused women than physical abuse.
Psychological abuse has been more strongly correlated with PTSD symptoms than physical abuse (Taft, King, Murphy, Dedeyn, & Musser, 2005)
In one study, frequency and severity of physical abuse are NOT even significant predictors of PTSD, nor of whether women will leave a relationship BUT psychological abuse is a significant predictor of both of these (Arias & Pape, 1999)