The term dolly is ambiguous and can refer to several different moving devices. Dollies as sold in US hardware stores though are a small frame or platform with wheels that you would place under an object you've moving (example). Other things the term can refer to are hand trucks and platform trucks (also called a platform cart and other names).
The dolly as a small wheeled frame or platform are supported by dictionary definitions.
Merriam-Webster: 4a: a platform on a roller or on wheels or casters for moving heavy objects
Oxford English Dictionary: 2: A small platform on wheels used for holding heavy objects, typically film or television cameras.
Dictionary.com: 2: a low truck or cart with small wheels for moving loads too heavy to be carried by hand.
Wiktionary: 6: A small truck without means of steering, to be slipped under a load.