Used to describe to financial institutions whose primary business is trading. The opposite of buy-side, where financial institutions make investments either for themselves or on behalf of other investors.
The secondary market is where securities are bought and sold once they have been issued in the primary market. The secondary market gives a continuing opportunity for buying and selling and price discovery, and provides the liquidity that allows the primary market to function.
Costs that have already been paid for and can therefore be ignored in calculating the future profitability of a project.