The boldface terms (e.g. geysers, lava, electrons) appearing in the text have associated definitions which may be accessed in one of two ways. If your Web browser is JavaScript capable (and JavaScript is enabled), a popup window will appear when the mouse cursor is moved over the term. This window will remain on the screen until you close it, and will display the definitions of any other term over which you move the mouse cursor. If JavaScript is not enabled or not supported by your Web browser, you may click on the term to display the definition in a new browser window, which may be used for all subsequent definition displays. This window may be resized and moved as you wish.