Controller
Short definition
The controller is the person or body that, alone or jointly with others, determines the purposes and means of the processing of personal data (GDPR, Article 4(7)).
In plain language
It is the party that decides why and how data are processed, and the one primarily accountable for complying with the GDPR. In a hospital or residential care home, the institution is usually the controller of its service users' data. If it hires a supplier that processes those data on its behalf, that supplier is a processor.