I've registered to this board specifically for this problem, though I'm beginning to suspect that I'll be here MUCH more often.

Recently I've tried to add a Lightbox script to an already existing page that already had a javascript feature in it. Now I know that there's no limit to the number of scripts you can run simultaneously, but when I added the lightbox feauture, my compiler gave me the following error:

$ is not defined
this.elements = $(this.options.thumb...lements(this.options.itemsSelector);

This line of code was part of the original source code of the page, and hadn't been a problem before I added the Lightbox code. Why is $ undefined all of a sudden and where can I find a solution for this? Could someone point me in the right direction here? I'm not exactly a novice(yet) when it comes to Javascript and this project was due yesterday.
I'd appreciate it if you guys could help me.