A spike is a comparatively large upward or downward movement of a price in a short period of time. Spike also refers to the trade confirmation slip which shows all the pertinent data for a trade, such as the stock symbol, price, type and trading account information.
A good example of a negative spike in the financial markets is the infamous stock market crash of Oct. 19, 1987, when the Dow Jones Industrial Average DJIA plunged 22% in a single day. [edit: another is the 2010 Flash Crash - https://en.wikipedia.org/wiki/2010_Flash_Crash]
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