For investors, things are great when the prices of their investments are moving in an upward direction. Inevitably, though, the value of those investments will dip at one point or another. This is the nature of the market: ups and downs. A sell-off, however, is different. It is the accelerated selling of securities, including stocks, bonds, commodities and currencies, beyond the daily ebb and flow of market prices. 

The ability to identify a sell-off can be an extremely reliable resource to have in a time of market uncertainty. These signals can present themselves in various forms so be prepared to analyze a number of indicators.

Source : Investopedia