Abuse is anything that causes harm to an individual. Abuse can be physical, sexual, psychological/emotional, neglect or economic/financial. Abuse of people with disabilities, like all forms of abuse, is an abuse of power and control.
Abuse of people with disabilities can involve systemic abuse which can happen in settings where other people are making decisions for the person who has a disability. Government bodies and bureaucrats can also be involved in systemic abuse. Institutional abuse is a form of systemic abuse in institutional settings where power imbalances often exist between service providers and people they serve.