Mark Price
The reference price a derivatives venue uses to value open positions and decide liquidations, drawn from several outside spot markets.
Rather than valuing positions at the last trade printed on its own book, a venue computes a mark price from an index of external spot venues, often adjusted by a component reflecting the contract's funding or basis. This makes it much harder for a single large order to trigger liquidations by pushing the venue's own price briefly away from the wider market. Unrealized profit and loss, margin calculations, and liquidation triggers all reference the mark price, which is why a trader can see a sharp wick on the chart without being liquidated. Each venue publishes its own index composition and update rules, so the same position can be marked slightly differently across venues.
In practice
A brief spike on one exchange's own order book may not affect positions there, because margin is calculated from an index of several outside spot markets instead.
The common misunderstanding
Positions are not liquidated because of the last trade printed on the venue; the mark price, averaged from outside markets, is what decides.