When it's mentioned in a sale then most likely they are referring to how many times the particular term was searched for in a month. You can find the tool at http://inventory.overture.com that will let you know the amount of searches last month for a particular term or a name w/ extension. :tu:
I believe Overture only gives results for Yahoo and MSN. As far as I know there is no equivalent tool for Google. If this is true I would think an overture search of 50,000 for example would be more than doubled with the addition of the popular Google search included.