Cacher is the code snippet organizer for pro developers

// source: http://bradsknutson.com/blog/highlight-search-terms-wordpress/
function highlight_search_term($text){
    if(is_search()){
  	$keys = implode('|', explode(' ', get_search_query()));
		$text = preg_replace('/(' . $keys .')/iu', '<span class="search-term">\0</span>', $text);
	}
    return $text;
}
add_filter('the_excerpt', 'highlight_search_term');
add_filter('the_title', 'highlight_search_term');