flattener

Etymology

From flatten + -er.

noun

1. Anything that flattens.
2. (finance) A trade whose terms are based on the assumption that the rate of return will decrease.
   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
   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