Behold Wall Street's new bull market, maybe

S&P 500 (.SPX) advanced on Thursday, putting the benchmark index up 20% from its Oct. 12 closing low and heralding the start of a new bull market, at least by the definition of some market participants.

Part of the uncertainty is that there is no set definition of a bull or bear market, or any sort of regulatory body that declares one, such as the National Bureau of Economic Research (NBER) does with recessions.