What is Blockage Discount?

Legal Definition
Blockage discount is an art-business-related and legal term of art for referring to the money discount assigned to a group of artworks by a single artist when that group of works is to be released to market as a group rather than individually. A blockage discount adjusts the fair market value of the works downward because of the risks of depreciation when a large volume of art is released into the market all at once.
-- Wikipedia
Legal Definition
Reducing the price of a block of stock. The value has gone down due to its lack of selling in the current market.