A spike is a non-market quote.
A spike may look like this on a chart:
Signs of a non-market quote include but are not limited to:
A significant price change within a few seconds
The price returning to its initial level
A lack of specific news
The primary reason for non-market quotes is a technical error. For example, a liquidity provider may send a broker the following price flow: "99.00; 100.00; 100.00; 101.00; 100.00; 99.00". Due to an error, the flow looks like "99.00; 100.00; 100.00; 10.100; 100.00; 99.00". As a result, the one-second chart shows a sharp downward movement to 10.100 from the 100.00 range (in this scenario).
Since the prices aren't market prices, any operations made at these prices are nullified, and the spike is removed from the chart.