Source : Free On-Line Dictionary of Computing

ISINDEX
     
         An {HTML} tag which tells the {browser} to
        display a text entry box on the current page.  Any text
        entered in the box by the user is appended as a URL-encoded
        query string to the current {URL} and sent to the {server}
        using a GET method.
     
        This is a simple way of making a {web site} searchable or
        allowing other kinds of simple user input.  It relies on the
        server mapping the query URL to an appropriate process,
        probably depending on the page in which the ISINDEX appeared.
        More complex input can be catered for using the {FORM} tag, or
        {Java}.
     
        (1996-12-22)