How to output a Rich Text Link to an Entry as a tooltip?
Not really a craft solution — but easiest might be jquery/ajax. Since you know the base URI, you can just create a selector for the route, and work from there.

$("a[href*='route/to/definitions/']").each(function(){
    var $element = $(this);

    // ajax load content and place content in DOM somewhere
    $.get( $element.attr('href'), function(data) {
        $element.attr('data-definition') = data;
    });

    // click handler to prevent default link behavior
    $element.on('click', function(event){
        event.preventDefault()
    });

    // hover handler to display/hide definition
    $element.hover(displayDefinition, hideDefinition);
});


You could also potentially do a search/replace in twig, but it is not nearly as easy to manipulate as in js.