flattener

plural flatteners

1. Anything that flattens.
2. businessA trade whose terms are based on the assumption that the rate of return will decrease.

   For example, if a trader thought that 2's 10's would trade lower than 185, he or she would put on a flattener.

   2014, Greg Gliner, Global Macro Trading: Profiting in a New World Economy

   1 more example

   Provided that the curve doesn't ever flatten too much and the option remains out-of-the-money, then we will put on flatteners when the curve steepens and put on steepeners when the curve flattens.

   2012, Howard Corb, Interest Rate Swaps and Other Derivatives, page 451