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:
The primary reason for non-market quotes is a technical error. For example, a liquidity provider sends 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 1-second chart shows a downward movement.
Since the prices aren't market prices, any operations made at these prices are nullified, and the spike is removed from the chart.