HTTP verb info: PATCHSource:
The PATCH method is used to apply partial modifications to a resource.
The PATCH method requests that a set of changes described in the request entity be applied to the resource identified by the Request- URI. The set of changes is represented in a format called a "patch document" identified by a media type. If the Request-URI does not point to an existing resource, the server MAY create a new resource, depending on the patch document type (whether it can logically modify a null resource) and permissions, etc.
See https://tools.ietf.org/html/rfc5789#section-2 for further details.