What is a "rare" item?

I recently bought an Invader Zim picture disc(a 12” vinyl disc) with the theme song on one side and Gir’s “Doom song” on the other. The seller said it was a “rare” item, and I have to admit, I haven’t seen many of them. I just wonder, are all the items that people claim to be rare actually rare or is it just a trick to sell the item(either way, the disc was $7 with shipping and handling so I really didn’t mind buying it, after all, it’s Invader Zim)?

Anyway, I’m just wondering whether items like the vinyl are actually “rare” or not?