Updated: Jul 12
Last Price is the last price of any trade what takes place at Bitmex platform. We count even 1 USD value trade. Important guide is that Mark Price (fair price) used at Bitmex platform it is a price computed as average price from tree other cryptocurrency exchanges: Kraken, GDAX (coinbase), BitStamp + orderbooks. Why do we need to use two different prices for?
Reason why we have Last Price and Mark Price at Bitmex? To Avoid price manipulations. Imagine the situation when you are shorting and somebody else clicked with fat finger to buy 1,000,000 contracts instead of 100,000 Let say just by mistake.
Average Bitcoin price on main exchanges is in 8000 - 8010$ range.
Bitmex Mark Price = 8005,5$
Bitmex Last Price = 8007,5$
Your are short and your liquidation price is 8030$. You are about to close it by clicking Buy Market but some fat finger at Bitmex has just brought 1.000.000 and it dramatically increased Bitmex last price up to 8335$ (for one second) and after while last price is 8008$. Notice that 8335$ is higher that your liquidation price and it means your position would get liquidated. It would but not will only because Mark Price protected your trade. Generally Mark Price is in trader's favour. The point is that Mark Price is more independent than Last Price.
All above is about Bitcoin and Ethereum Perpetual contracts. Computing Mark Price for futures contract is a little more complicated as there is value in time added as well - look more guides and tutorials on MinistryOfMarginTrading.