A node is a physical machine in a topology comprised of one or more machines. A node contains a WebSphere Application Server installation.
IBM WebSphere Application Server Advanced Single Server Edition resides on a single node. That is, each machine holds a separate installation of the product that is unaware of installations on other machines.
IBM WebSphere Application Server Advanced Edition provides centralized administration of multiple nodes containing WebSphere administrative servers. That is, the WebSphere administrative server on each machine (node) can be configured to use a shared administrative database.