◇ GLOSSARY
Mark price
The exchange's fair-value estimate for the contract, typically built from the index price plus a decaying basis component rather than the last trade. Unrealized P&L, margin checks, and liquidations all key off mark. A single wild print on last does not liquidate you; a move in mark can.
Mark is the venue's answer to a thin book wick. Last is whatever traded last; mark is index plus a smoothed premium or discount so one fat-finger fill does not wipe half the open interest.
Your position panel uses mark for unrealized P&L. The liquidation engine uses mark for maintenance checks. Twitter uses last. When they diverge 0.3% in a fast market, you can be green on the ticker and closer to liquidation than you think.
Do not debate which is fair in abstract. Know which one your venue uses before you size. On most perp books, mark is the number that forces you out.