What is Fortune 1000?

Legal Definition
Fortune 1000 is a reference to a list maintained by the American business magazine Fortune. The list is of the 1000 largest American companies, ranked by revenues. Eligible companies are any which are incorporated or authorized to do business in the United States, and for which revenues are publicly available (this list has a larger universe than "public companies", as the term is commonly understood, meaning "companies whose common stock trades on a stock market"). The Fortune 500 is the subset of the list that is its 500 largest companies.

The list draws the attention of business readers seeking to learn the influential players in the American economy and prospective sales targets, as these companies tend to have large budgets and staff needs.

Wal-Mart Stores was number 1 on the list for five of the seven years from 2007–2014, interrupted only by ExxonMobil in 2009 and 2012.
-- Wikipedia
Legal Definition
The top 1000 largest US public and privately-held companies are listed based on reported revenues. It is an annual ranking by Fortune magazine.