By TheStreet Staff What Is a Stock Market Crash? It’s every investor’s worst nightmare: a sudden and steep market decline that wipes out years of gains in a matter of days. They affect everyone from retail investors to professional traders to institutional fund managers to those saving for retirement, to name just a few. Stock market crashes are not that common; they often catch investors by surprise, but when they do happen, they result in widespread losses that spell even further downside—and it can take generations to recover. A stock market crash is characterized by a decline of at least 1…