mortgage originators can either hold loans in their portfolios or sell them to investors. When a mortgage originator decides to sell mortages to another institution, this transaction occurs in wehat is commonly reffered to as

Secondary mortgage market

Explanation:

Secondary mortgage market is the market where the primary mortgage is sold to other party.

As in the mortgage there is lending of money as against some property, and that the lender then sells such right to receive such money back, to some third person, which basically means selling of right to receive payment.

And then further transferring the mortgage is the secondary market transaction.

As sometimes the lender needs more money but the mortgage is not due, in that case he sells such mortgage.