A Spike is a swift market move that quickly traverses several price levels.
Spikes are caused by Aggressive Participants who are eager to have their orders quickly executed at the best available market price. Spikes occur when Market Order(s), often as stop orders, overwhelm the available liquidity in the Order Book. Price moves quickly through several levels before the Market Orders are exhausted and the opposing participant aggressively seizes upon the opportunity to execute at what they perceive to be advantageous prices, bringing the market back to where the move began.
Spikes may be a sign of elevated probability for some kind of reversal in price. They can be observed in all markets and time frames, but their size and impact will vary depending on instrument, liquidity, participants involved, etc