Icon Communications / Contributor A stock market crash is a sudden or severe drop in overall share prices, usually within a day. Stock market crashes can be due to economic or natural disasters, speculation, or investor panic. Investors can prepare for stock market crashes by diversifying portfolios and shifting to CDs or bonds. The stock market is constantly moving, prices of individual equities rising and falling throughout the trading day. Whenever the majority of them — or a representative group of them, called a stock market index — takes an especially large dive, a panicked cry often arises: "The stock market has crashed!"Stock market crashes are certainly scary: Hundreds of investments decline their value, investors lose thousands of dollars — on paper, anyway.