flattener

Anything that flattens.

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.

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