A Project object in Paper.js is what usually is referred to as the document: The top level object that holds all the items contained in the scene graph. As the term document is already taken in the browser context, it is called Project.
Projects allow the manipluation of the styles that are applied to all newly created items, give access to the selected items, and will in future versions offer ways to query for items in the scene graph defining specific requirements, and means to persist and load from different formats, such as SVG and PDF.
The currently active project can be accessed through the paperScope.project variable.
An array of all open projects is accessible through the paperScope.projects variable.