The term node is a catch all term for any content form in use in a Drupal site. So this means that a Page is a node, so is a Story, and a Book page - and any other content type that might exist in Drupal.
Why is the term node used? There are actually a couple of really good reasons for this...
First, a node, in mathematical terms, is a point where several connecting lines come together - like a point on a map. If you were to create a map of a web site, you would see that the web "pages" are usually connected, and often times they may be connected in many different ways. Semantically speaking, the term node is actually a much more precise term than "page".
Second, in a Drupal site, one of the basic content types is called the Page content type. Referring to all nodes as Pages to a Drupalista may cause some confusion - especially if working on a site where the Page content type isn't even used.