By TheStreet Staff What Is a Bear Market Rally?Bear markets—typically defined as sustained, market-wide downturns of 20% or more as measured by a bellwether stock index like the S&P 500—are a natural part of the investing world. They help bring overvalued stocks back to earth, and they set the stage for future gains. Just as natural are bear market rallies—shortlived recoveries of 5–10% that occur during the midst of a bear market before stocks fall to new lows. Similar to a dead-cat bounce, a bear market rally is a brief period of optimism that drives prices up temporarily before bearish sent…