Here's a definition:
Same-store sales, or "comps," measure sales growth at store sites that have been open for more than a year. So, for a store to be included in a company's monthly comps for May 2000, it must have been open for the full month of May 1999. If the store opened May 15, 1999, it would not be included in the company's comps until June 2000, a year after the store's first full month of operations.
http://www.fool.com/news/indepth/ret.../samestore.htm