A yield-rate environment in which short-term interest rates are increasing at a faster rate than long-term interest rates. This causes the yield curve to flatten as short-term and long-term rates start to converge. 
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At any time, the yield curve is either in a state of steepening or flattening. These fluctuations occur due to investor demand, change in interest rates, and institutional investors trading large blocks of fixed-income securities. If the curve is flattening, the spread between long-term rates and short-term rates is narrowing. A bear flattener often occurs when the government raises interest rates in the short term. Increasing interest rates drives short-term bond prices down, increasing their yields rapidly in the short term, relative to long-term securities.