A widening of the yield curve caused by long-term rates increasing at a faster rate then short-term rates. This causes a larger spread between the two rates as the long-term rate moves further away from the short-term rate.

Investopedia Says:
This widening yield curve is similar to a bull steepener except with a bear steepener this is driven by the changes in long-term rates, compared to a bull steepener where short-term rates have a greater effect on the yield curve.

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