When a trade in a listed stock is not executed through the listing exchange.
Investopedia Says:
Historically, in an upstairs trade buyers and sellers would negotiate the price and conditions of the trade in the "upstairs" rooms of a brokerage firm. Some exchanges prohibit upstairs trading.
Related Links:
Find out the answers to all the questions you had about stock exchanges but were afraid to ask! Getting to Know Stock Exchanges
Knowing how the primary and secondary markets work is key to understanding how stocks trade. Markets Demystified
Listed securities are 'the cream of the crop'. Find out how a firm can lose that status and why you should be wary. The Dirt On Delisting
